Triplescreen setup - how to correctly?

is there any guide/way how to setup triplescreen (aka surround for nvidia). I have three displays, I am using KDE DE, nvidia setup shows one X Window with resolution for all three, but they still acts separately - in game can only select resolution equal to one monitor and also the games run in borderless fullscreen on one monitor. I found some info regarding Xinerama, but it did not help.
The goal is to a) be able to select resolution across all screen ingame (for some games) and be able to play others on the middle one in fullscreen (and turning off/black the others). I am struggling even for native games like ETS2.
So what is correct way how to setup surround in linux?
Thanks a lot for help!

So, two completely different modes of operation:

  1. one display across all three outputs, and
  2. one display on a single output

I suspect you want to use the DE’s display settings for the second use-case, and for the first one combine them into a virtual display:

Yeah, I tried that…but technically I see it setup like it, but still the games cannot see it, they just see one screen 2560x1440 resolution. As mentioned before I see X Screen 0 with resolution 7680x1440 and then three screen 2560x1440, but I was not able to propagate it to games somehow.

EDIT: Maybe my only issue is how to switch/choose them.