Latitude 491, WD19DC and 3 external monitors

@trygveah  There are at least three problems here.

First, there is no "Latitude 491" system.  I'm not sure what model you meant to type there.

Second, if you're using only one connector of the WD19DC because whatever Latitude system you have only has a single USB-C/TB3 port, then the WD19DC behaves functionally equivalent to the regular WD19.  And a regular WD19 when connected to a system that only supports DisplayPort HBR2, which might be the case with your system, only supports dual displays up to 1920x1200 each or a single QHD/1440p display (or 4K but only at 30 Hz).  This is due to bandwidth limitations on USB-C when the link is set up to carry both video and USB 3.x data, as a dock would set it up to do, and when the system only supports DisplayPort HBR2 for video.

And third, the WD19DC itself can support 4 displays -- but that doesn't mean the system can.  Any Latitude system ending in 491 would be a 14" system and therefore I believe would only ever have an Intel GPU.  Even if it did have a discrete GPU, on almost all Dell systems, the USB-C/TB3 port is still wired to the Intel GPU.  And as of this writing, Intel GPUs only support running up to 3 simultaneous independent displays total.

So while I can't be sure without knowing the actual Latitude model you have, it seems somewhat likely that:

• You've got a WD19DC that is offering you no benefit because your system can't take advantage of its dual connector capability, and is therefore operating like a much less expensive WD19.
• You've got bandwidth limitation issues due to only having DisplayPort HBR2 support and running in "regular WD19 mode", which will prevent you from running 3x 1920x1200.
• You wouldn't be able to run 4 displays anyway, even if the dock itself supports it.

If your system has Thunderbolt 3, then a WD19TB would allow you to run 3x 1920x1200 displays as long as you disabled the built-in panel before lighting up the third external display.

And by the way, the documentation of the WD19 dock family clearly states that the HDMI and USB-C outputs cannot be used at the same time, so you would not be able to run 4 displays that way.  You'd need a DisplayPort daisy chain to run 4 (or the "downstream TB3" port on the WD19TB).

The system is a E5491.  

sorry about that. I have a Plastic Fantastic DELL(KB212-B) keyboard, and if I type quickly it sometimes doesn't register certain keys...  

We only have the TB16 /TB18, the WD19DC, and the DC6000 docks at the office, for use with more modern DELLs.

Standardisation. I didn't decide on which docks to get. I'm just trying to get them to work with the user's setup.

I could setup one of the TB16s again, but the users hate those more than the plague, and frankly, so do I. 
(I do user support, and I have lost count on how many times I've talked a user through power resetting those. Even when they have the latest FW. )

@trygveah  If you've been ordering your Latitude 5491 systems with Thunderbolt 3, which is optional, then you might want to consider the WD19TB.  It's the replacement for the TB16 and it's been getting FAR fewer complaints about problems.  And it hasn't had the flurry of firmware and driver updates that the TB16 always had.  I agree that the TB16 had far more than its fair share of problems.

The WD19TB when used with a Latitude 5491 that has TB3 would be able to run 3 displays up to QHD/1440p each.  You didn't specify what resolution(s) you're trying to run.  Also note that depending on the resolution in question, you might have to connect one of those displays to the "downstream" TB3 port on the dock -- not to be confused with the USB-C output near the HDMI output.  (This port doesn't exist on the WD19DC since it's not a Thunderbolt dock.)  This has to do with how the dock allocates display bandwidth from the system across its outputs, and even that varies based on whether the system supports DisplayPort HBR2 or HBR3.  I'm 95% certain that the Latitude 5491 would be an HBR2 system.  You may want to consult the Display Resolution Table in the WD19TB's manual available on support.dell.com for more details on capabilities and cabling restrictions.

If you don't have Thunderbolt, then the WD19TB would operate just like a regular WD19 (or a WD19DC running in single cable mode), which won't be enough for triple displays of any meaningful resolution.  In that case, the only way to run triple displays at 1080p or above would be with the Dell D6000, but that dock uses "indirect display" technology called DisplayLink (not to be confused with DisplayPort).  That will allow triple QHD 60 Hz displays or dual 4K 60 Hz displays plus a single 4K 30 Hz display even without Thunderbolt, but the compression technology that DisplayLink uses in order to achieve that comes with some drawbacks that I've written about as the post marked as the answer in this thread.  Also note that the D6000 only supplies 60W to the attached system, which might not be enough to run your systems properly.  Check the wattage of the power supplies that came with your systems to find out.  The WD19TB can supply up to 130W to the attached system.

@BLichtenwal  Happy to help.  In terms of your setup, there are definitely problems here.

First, the WD19DC serves absolutely no benefit to an XPS 15 7590.  It will operate just like a regular WD19 180W, so if that's all the functionality you need, you could have saved some money there.  But you want to run 3 external displays.  Unfortunately you didn't specify their resolution (always a good idea, especially since you already know bandwidth is a potential concern), but a regular WD19 and therefore a WD19DC operating in single connector mode as in your case only has enough bandwidth for dual displays up to 1920x1200 each, or a single display up to 2560x1600.  So if there's a way to run triple displays, they would have to be pretty low resolution and/or refresh rate (I'm assuming the standard 60 Hz refresh rate here).

If you want triple displays all through a dock, the dock you really need is the WD19TB.  That's also a single connector dock, but it uses Thunderbolt 3 rather than USB-C, and therefore when paired with your system it can tap into 4x more display bandwidth than the WD19 (or the DC running in single connector mode).  As a result, it can run dual displays up to 4K 60 Hz each or triple displays up to 2560x1440 each.  Note that with the WD19TB, in either of those cases you will need to connect one of the displays to the "downstream TB3" port at the edge of the dock, not to be confused with the USB-C port near the HDMI output.  The WD19 and WD19DC don't have this port at all, but the WD19TB does.  This requirement stems from how the dock allocates video bandwidth received from the system, which varies based on the system's capabilities.  But what I just said is correct for the XPS 15 7590.

If on the other hand you didn't want to get a new dock but were ok using the HDMI output, then the 7590's HDMI output supports HDMI 2.0, which means it can run up to 4K 60 Hz.  If you connect a display that way, then that display's bandwidth requirements don't have to be met through the dock connection.  So if you keep the WD19DC, then as long as the 2 displays connected to that dock didn't have resolutions higher than 1920x1200 (again assuming the standard 60 Hz), that setup would work.  But of course that's a bit less convenient.

But your question about running the built-in display brings up the second problem here.  The XPS 15 7590 has all of its display outputs driven by its Intel GPU, even if you have an NVIDIA GPU installed.  The Intel GPU in that system is limited to 3 displays total.  So if you want to run 3 external displays, you'll have to keep the built-in display disabled.  That is true regardless of how the displays are connected because that limitation exists on top of any other limitations that may exist around bandwidth.  If you want to run more than 3 total independent displays, the only option would be to connect one of them through a dongle or dock that used "indirect display" technology such as DisplayLink -- not to be confused with DisplayPort.  But DisplayLink has some limitations that can be significant in certain use cases, especially on systems that have dual GPUs like yours if you have the NVIDIA GPU.  I wrote about those in the post marked as the answer in this thread.  If those drawbacks might be acceptable to you for at least one of the displays you'd be running, then you can get a USB 3.0 to HDMI or USB 3.0 to DisplayPort adapter that uses DisplayLink and then just connect it to a USB 3.0 port on your dock.  Since it uses DisplayLink, it won't count toward your GPU's display max and won't count toward the dock's video bandwidth limit since it uses USB data instead.

If on the other hand you need triple external displays plus your built-in display and DisplayLink's drawbacks aren't acceptable to you, then unfortunately you've got the wrong system for your purposes.  The only laptops that can run 4 total displays all via native GPU outputs are systems that have at least some display outputs wired directly to a discrete GPU or systems using Intel's very latest Xe GPU built into its 11th Gen CPUs.  I myself have a ThinkPad X1 Extreme Gen 2, which is the only real non-Apple competitor to the XPS 15.  It has its video outputs wired to the NVIDIA GPU and has the built-in display by default controlled by the Intel GPU, though that can be switched to the Intel GPU too.  But in its default mode, since the NVIDIA GPU on its own can run 4 displays, I can actually get 4 external displays and the built-in display all running at the same time -- even though that's not an officially supported configuration because achieving that requires some care in balancing bandwidth concerns and GPU interface allocations.  But I've done it.

@jphughan Thanks for all your detailed explanations of this scenario. I have a similar issue and want to confirm I understand my limitations. My system is:
- Laptop - XPS 15 7590
- Dock - WD19DC
- O/S - Windows 10
- All firmware up to date

I want to use the laptop screen, plus 3 external monitors via the dock (2 on DP, 1 on HDMI). However, I can not get Windows to extend the desktop to more than 2 external monitors. As soon as I extend to a third monitor, it disables one of the other 2. 

It sounds like the limitation is due to the XPS only having 1 USBc for the dock to connect. As a result, there's not enough bandwidth to run 3 screens from the dock.

Questions:
1. Is my understanding correct?
2. Is there any way to get the HDMI on the XPS to run one, while still using the XPS screen and the dock runs 2?
3. Is there any alternative to get 3 external monitors working while also using the laptop screen, without more hardware?

Thanks so much for your support!

@jphughan Wow. Again, I am grateful for the incredible amount of detail and attention. Let me know if I can treat you to a beverage. One more follow up question....

I do have the NVIDIA GPU. My external screen resolutions are:
- 2560x1080 (primary)
- 1280x720 (teleprompter)
- 1280x600 (used for slides)
Odd, I know, but it's what I need to fit my teleprompter and space when giving virtual presentations. 

Given these resolutions and your detailed response, I understand I have two possible solutions:

1. WD19DC + DisplayLink
The only way to get 3 external + the laptop screen running is through a USB to HDMI (or DP) cable, using DisplayLink.

2. Getting a WD19TB
If I get the WD19TB dock, instead, will it support 3 external + the laptop screen without having to do Displaylink? 

I think running displaylink through the telemprompter should be fine. But I obviously prefer to minimize cpu processing during presentations.

@BLichtenwal  Happy to help!

Neither the WD19, WD19DC, nor WD19TB will allow you to run more than 3 total displays plus the built-in display.  The WD19TB only increases the amount of bandwidth available to the dock, which allows running higher-end displays and/or more displays through the dock than might be possible with the WD19, but it doesn't change the fact that the GPU still has a hard limit at 3 displays, regardless of bandwidth, how displays are connected, etc.  The fact that you have an NVIDIA GPU doesn't change anything with this particular system because all display outputs are still wired to the Intel GPU anyway, so you can't split displays between those GPUs.

Looking at total video bandwidth available from your system to the WD19DC and the bandwidth required by the resolutions used by your displays, the math suggests that even the DC operating in single connector "regular WD19 mode" would be able to access enough bandwidth to run all 3 of them, thanks to the relatively low resolutions of your "auxiliary" displays.  But first, not everything that should work on paper actually does work in the real world.  And second, even if it does work, you'll still be left unable to use the built-in display in that setup.

So yes, if you want to run all of that, you'll need one of your displays to be run from DisplayLink.  Any 2 other displays in your setup should definitely work fine through the DC using regular video outputs.

Note that Dell does have a dock called the D6000 which can support up to 3 simultaneous displays, with 2 of them driven via DisplayLink.  As a result, that single dock could run all 3 of your external displays and still allow you to run the built-in display.  The catch is that the D6000 isn't a great choice for the XPS 15, since the D6000 only supplies up to 60W over power, which is less than half of the 130W required by the XPS 15.  So with that dock you'd either have to keep the XPS 15's power source connected separately for optimum performance or put up with slower battery charging and reduced performance that will result from running the XPS 15 from a 60W source even though it's designed for a 130W source.  And then you'd have DisplayLink's potential drawbacks on two of your rather than just one.

But if there's any way you can return/exchange the WD19DC, I really would, because it just doesn't make a ton of sense for your system.  If you don't need more video bandwidth, then the WD19 180W will work exactly the same way as the DC with your system at a lower price.  And if you do need the extra bandwidth (or want it for futureproofing), then the WD19TB will allow you to use 4x more bandwidth and might still be less expensive than the DC.