Your question about whether it works is really three different questions:
1. Can qubes support 3 monitors?
2. Can qubes support 4k?
3. Can qubes support different resolution.
I don't know the answer to 1, although I can affirm it definitely supports 2;
note that there may be limitations with the frame buffer size but X supports
multiple graphics cards.
In response to 2, it works well on a 4k display (I'm typing it on it now).
In response to 3, X thinks all your displays have the same density, i.e.,
dots per inch. If you don't, it may be painful if you want to avoid tiny
or gigantic type. If you search this group you'll get some help for this
problem.
Jon
I run 1600x900 and 1920x1080 on this PC. I have one PC that has 6 monitors and it works just fine.
So yes, it will all work fine. It's Linux. Then again it is Fedora with SystemD, so it will most likely crash and stuff up for 4K just because it feels like it.
If it was a SystemD free version of Linux, it would have 0 issues.