Hi, have 5 monitors and i run Ubuntu 19.10. My GPU’s max is 4, but it’s not a problem since i need 3 maximum active simultaneously. Even if i have settings working (other 2 disabled) in the login screen and in the session, during the transition output is pushed on all monitors attached (and of course this causes a crash, since GPU can’t handle… ).
Try disabling xinerama. I have 3 monitors running, but only one desktop is operational. The other 2 only show a black screen and “X” as a cursor. Unable to drag other apps to these monitors. Curious how you were able to get your 3 working (even if momentarily). Are you running X-server?
Well, in reality i don’t use Xinerama, but X-server. This because i need 3 of them for working, so multitasking it’s what i need. Have a GTX 1070 and can do even 4 (its maximum) at the same time if i want, but don’t use. Found that the issue with nvidia drivers happens with GNOME, since on XFCE 5 are detected but then only 3 are used. If i unplug the 5th i can use all the others normally like on Windows, not momentarily and can drag other apps to other monitors (IDE, docs, device/program in question etc.). Without nvidia drivers no problems. Now i wait for an answer here, if don’t receive a solution from GNOME team. Hoping to not debug all GDM stuff and whatever…
Anything under maximum of the GPU can be handled easily with inbuilt display settings of Desktop Environment.