Dell Precision 5550 and external display

@derhenni96 Happy to help. HBR2 vs. HBR3 isn't your issue here.  That only affects the amount of bandwidth you can send to external displays -- the dock, in this case.  You have enough bandwidth with a TB3-capable HBR2 system to run the external display setup you want to run.  The limitation you're running into is the number of displays supported by the GPU that's wired to all outputs.  That's an entirely separate thing.

But yes, Dell's documentation (and that of other vendors) do not often specify the DisplayPort revision supported by the system.  They specify on their displays and docks what setups are possible from systems that support each type, but not in the documentation of their systems.  I've pointed this out to an internal Dell contact, who relayed it to the Documentation team.  It seems to be getting better, because for example the XPS 15 9500 and 9700 specs identify a specific DisplayPort revision rather than just saying "DisplayPort" -- but there are errors on both of those.  The XPS 15 9500 documentation incorrectly claims that the USB-C port on the right supports DP 1.4 (it's only 1.2) and the XPS 17 9700 documentation says it only supports DP 1.2 even though if you order it with an RTX GPU and enable a BIOS option to have the NVIDIA GPU control the outputs, it will run DP 1.4.  So sort of a one step forward, one step back situation there.  More information, but not completely accurate information.

Speaking of the XPS 15 9500, no that won't solve anything because that's essentially identical to the Precision 5550 except it uses a GeForce GPU rather than a Quadro GPU.  But the Intel GPU controls all outputs in that system as well.

The X1 Extreme and P1 are different because the NVIDIA GPU has direct control of the display outputs, and they support 4 total displays.  And then those systems have a BIOS option allowing you to choose whether the Intel GPU or NVIDIA GPU should control the built-in display.  If you keep it at the default Intel GPU setting, then you can run 4 external displays plus the built-in display.  That isn't actually noted or officially supported because actually achieving that requires careful balancing of bandwidth, ports, and source GPU interfaces, but I've personally done it with my X1 Extreme.  But 3 external displays plus the built-in display is completely supported.

So if you want a system that can run 3 displays, you'll either need a system like that where the NVIDIA GPU controls at least the display outputs, or you'll need a system with the new Intel Xe GPU, which supports 4 simultaneous displays.  The Core 11th Gen CPUs seem to have them, such as found in the XPS 13 9310.

In terms of HBR2 vs HBR3 support, if the discrete GPU controls the outputs, it's overwhelmingly likely that it supports DP 1.4/HBR3.  If the Intel GPU controls it, then Intel GPUs that support DP 1.4/HBR3 include the Ice Lake CPUs, which are certain Core 10th Gen CPUs (the ones ending with a "G" followed by a number, NOT the ones ending with U or H) and then all Core 11th Gen CPUs, at least as of this writing.

The problem with systems that have dual GPUs is that the documentation practically never says which GPU controls the display outputs.  But as I said, the Precision 5550 and XPS 15 9500 both have the Intel GPU controlling them.  The XPS 17 9700 if you order an RTX GPU -- not a GTX GPU -- has a BIOS option allowing you to choose which GPU controls them.  The Precision 7000 Series models also have that option, along with an option to specify which GPU controls the built-in display.  And the X1 Extreme and P1 always have the NVIDIA GPU controlling the outputs with an option to choose which GPU controls the built-in display.

If you're curious, I wrote a comparison of the XPS 15 9500 vs. X1 Extreme Gen 2 here.  The same logic applies to the Precision 5550 vs. P1 if you need a Quadro GPU.  The X1 Extreme has moved to Gen 3, but the differences are minor: proprietary Ethernet connector dropped, option for built-in LTE, option to order an Intel-only config (no NVIDIA GPU, but in that case it's an HBR2 system limited to 3 displays), and I think the display options changed a tiny bit.

Hopefully this helps!