Connect latitude L7300 to a 4K at 60Hz external monitor (U2720Q)

@Zroove  I've got good news for you:

• Yes, that system is capable of running 4K 60 Hz on an external display.
• Technically you would be getting a USB-C to DisplayPort cable not a TB3 to DP cable since these cables don't tap into TB3, but yes a USB-C to DP cable will allow you to run 4K 60 Hz, as long as you connect it directly to the USB-C port on the system and not through any sort of multi-purpose hub or "mini-dock" device.
• You can use dual 4K 60 Hz, but not with the WD15.  First of all the WD15 has been replaced by the WD19, but since both of those are regular USB-C docks, they won't tap into enough bandwidth to run dual 4K 60 Hz.  Those docks with your system would actually only be able to do dual displays up to 1080p.  For dual 4K 60 Hz, you'd need an actual Thunderbolt 3 dock like the WD19TB, which when used with your system can tap into 4x more bandwidth than the WD15 and can therefore run dual 4K 60 Hz.  Note that with the WD19TB when using it with your system, one of the displays will need to be connected to the "downstream" TB3 port at the edge of the dock, using something like a USB-C to DP cable.  The other can be connected to any of the other outputs.  This requirement has to do with how the dock allocates display bandwidth from the attached system.  Or if you don't want an entire dock, there are Thunderbolt 3 to Dual DisplayPort adapters that could also run dual 4K 60 Hz, although they wouldn't provide the other functionality that a docking station would -- but they cost less.  You can find that type of product made by companies such as StarTech, Plugable, Sonnet, and OWC.  Just make sure you do NOT get a USB-C DisplayPort MST hub.  They look similar and they cost less, but since they don't use actual TB3, they won't tap into as much bandwidth and therefore won't run dual 4K 60 Hz displays.
• If you set your external displays to Extend rather than Mirror, then yes you'll be able to choose separate resolutions on each of your active displays.

@Zroove  The manual for the U2720Q available on support.dell.com indicates that the display will provide up to 90W of power when using a USB-C connection, which is plenty of power for a 13" Latitude would need.  But if you do that, then with your system if you wanted to run 4K 60 Hz, you'd only be able to carry USB 2.0.  Being able to carry 4K 60 Hz video and USB 3.x over USB-C would require a system that supported DisplayPort 1.4/HBR3 over USB-C, and yours only supports DP 1.2/HBR2.  Of course using a USB-C to DP cable would ONLY handle video, with no power or USB data at all.  Unfortunately if you wanted 4K 60 Hz and USB 3.0 with your system, then you'd have to use a USB-C to DP cable for video and then a USB-C to USB-A cable for USB 3.x data -- but then you're only using the display's USB-C port for USB data rather than power.  With your system and that display, there's no way to get power, USB 3.x data, and 4K 60 Hz all from the display at the same time.

Also note that if you use a USB-C connection to that display, you'd only be able to run one 4K 60 Hz display.  The HDMI output on your system is only 1.4, which isn't enough for 4K 60 Hz (you need HDMI 2.0 for that), so the only way you'd be able to run dual 4K 60 Hz displays is with one of the solutions I mentioned in my earlier replies.  If you use a TB3 to Dual DP adapter, you'll be able to run dual 4K 60 Hz displays from your USB-C/TB3 port, but you won't have USB data or power through that connection.  If you want dual 4K 60 Hz displays, power, and USB 3.x all over a single cable, you need a full Thunderbolt 3 dock like the WD19TB.