P2419HC, 3rd P2419HC low resolution 640x480

@dpetc  Just to clarify, it sounds like you're using a daisy chain involving DisplayPort cables, not DisplayLink.  DisplayLink is a completely different technology.

But if you're connecting to the first display using USB-C, then the display is setting up the USB-C link to carry both video and USB 3.x data simultaneously in case you want to use any USB 3.x peripherals plugged into that display's USB 3.x ports.  A USB-C link has four high speed lanes that can be allocated to video or USB 3.x (or Thunderbolt 3 when supported, but that doesn't come into play here.)  Using USB 3.x requires two lanes, which means that it cuts your available video bandwidth in half compared to allocating all four lanes to video, as would be the case if you were using a USB-C to DisplayPort cable or something -- but then you'd lose USB 3.x data as well as power delivery.  Your Intel UHD 620 supports DisplayPort 1.2/HBR2 over USB-C, but when only two lanes are being used for video, you only have half of a DisplayPort 1.2 link's worth of bandwidth.  That is only enough for dual 1080p displays, plus a tiny amount of extra bandwidth that you're seeing as 640x480.  The reason the third display can run at full resolution over HDMI is because in that situation, the third display's bandwidth requirements don't have to be met from the USB-C link as it would have to be in the daisy chain scenario.

In terms of getting all 3 external displays working simultaneously and at full resolution, the Intel GPU you have only supports 3 simultaneous independent displays total, no matter what.  This limitation is separate from and in addition to any other limitations that may exist around resolution, bandwidth, etc.  If you're having trouble getting it to light up, try this:

• Open up your laptop lid.
• Set up a daisy chain of only two displays, connect them to your system, but leave the third display disconnected for the moment.  At this point you should have your built-in display and two external displays running.
• Go to Windows Display Settings and set your built-in display to "Disconnect this display".  It should go black.
• Connect the third display via HDMI.  It should now light up.

I've found that sometimes I have to explicitly tell Windows to disable the built-in display before it will let me light up a third external display.  Somehow just closing the lid doesn't seem to do the trick.  But the good news is that once you implement this configuration, Windows will remember that when those 3 displays are attached, the built-in display should be disabled, so it will do that automatically going forward.

Thank you for your thoughtful reply.   Yes, sorry, data link cables that came with the monitors.

I have tried your suggestion below.   And running the 3rd  monitor on HDMI was the way I had it previously.  When I connect it via HDMI it says "No HDMI Signal from your device"  I have tried 2 different cables same result.  I am able to run HDMI to the 3rd monitor if I unplug the USB cable and just run the 3rd monitor with my laptop

....and when I have the 3rd monitor connected just HDMI - it does not detect the 3rd monitor.  I have uninstalled and reinstalled the G620 driver; no changes.  The HDM/3rd monitor will light up if I turn off the 1st/USB-C connected monitor.  When I turn on the 1st/USB-C monitor the HDMI monitor goes blank again and says no HDMI signal.  So it seems I can't get output to both the USB-C and the HDMI at the same time.

@dpetc  In addition to the reply I just posted, I just reread your original post and you said, "Previously, I was able to get all 3 P2419HC to run 1920x1080. I did this by connecting P2419HC 3 to the PC with an HDMI Cable."  Do you mean you had all three of them running simultaneously at some point in the past, with one of them connected via HDMI?  In that case, my wiring theory wouldn't be correct.  But then my question is what changed that time when you were able to run two displays via USB-C and your third display via HDMI?  Any driver or BIOS updates that might have possibly introduced a new bug?

@dpetc  I'm not deeply familiar with the ThinkPad T480, but based on your description, it sounds like it might have a wiring design such that the USB-C and HDMI outputs share a common connection back to the Intel GPU, which means that only one of them can be used for display purposes at a time.  In that case the only way you'd be able to connect all 3 displays would be with a solution that tapped into the Thunderbolt 3 capabilities of your USB-C/TB3 port, in which case you'd have enough bandwidth for triple 1080p.  But the only "basic" adapters that do that focus solely on video output, in which case you'd lose the benefit of USB 3.x data and power delivery through that connector.  So the only way to run triple 1080p plus USB 3.x data plus power all over that single port would probably be with a Thunderbolt 3 docking station.

than you so much for your interest.

I solved the problem

There are 2 USB-C ports on the left side of the Lenovo.   I the USB cable plugged into the wrong port.  It should have gone into the thunderbolt port, which is closer to the keyboard.  They look alike. Now it works fine.  2 monitors on the USB, 1 on the HDMI and all in high resolution.  I took me all weekend to figure this out and I just had to swap the plugs.  OMG.  thanks again for your help.

@dpetc  Ha!  I've had stuff like that happen to me too.  Glad it was an easy fix!