I just installed 18.04 on my home PC yesterday - all is well, and for a while, it ran 3 monitors just fine.
But after a few hours of use today, one of the monitors went black, and disappeared from settings. If I plug / unplug cables, I can get other monitors to do the same thing, so it seems as if the Geforce 1060 is having a hard time driving three monitors (all running 1920x1080). Windows ran the three displays fine for a year, so the card /should/ be capable.
I then installed the latest ppa nvidia driver, and all three displays came back - but only for a while. After leaving the PC for a few hours to run some errands, it went back to running only two of the displays.
I've poked around here and elsewhere, but haven't seen this problem, but feel free to repoint me if my search-fu failed.