Having trouble with 3+ monitors with your 4080? Turn off DSC
For the past ~year, I've had this conundrum with my monitors where I couldn't enable more than two displays at a time. After enough searching, I saw a post about display stream compression enforcing some limit, but there was no setting for that in my monitors. Well today I realized there was a firmware update available for the monitors, so I installed it and noticed a "DSC" option in the menu. Turning this off for both monitors finally allowed me to use a third!
https://en.wikipedia.org/wiki/Display_Stream_Compression
I'm not entirely sure why this is on by default. After turning it off, I am now able to use both monitors at 1440p@240Hz in addition to my 1024x768@60Hz CRT monitor. This limitation apparently has something to do with a display buffer on the 4080, but I haven't dived deep enough to determine it.
Hopefully this helps another person who is having the same problem.