In x Server Display Configuration-Configuration there are three options and one of them is X screen 0
If you click the monitor image above you can select what kind of config you want.
I configure my three monitors using nvidia-settings with no manual intervention in /etc/x11/xorg.conf
I am running Leap 42.2 also.
Edit:
I see you are using two video cards for your three monitors.
I’m using just one video card for 3-monitors, sometimes I use 4 monitors but it
is too tight on my table so just using 3 now;)
I was having pretty much the problems described in this thread after upgrading to Leap 42.3 (from 42.1 which became unusable with my screen setup after an update) and was able to get things to work after trying out some of the suggestions here.
What finally worked was to designate both screens as screen 0 in nvidia-settings (I did this as root, but not sure if this matters) and unselecting ‘use Xinerama’. Having done this I was then able to use ‘System Settings -> Display and Monitor’ to activate and rearrange my two screens (of different size and pixel density). If I set up the screens as screen 0 and screen 1 in nvidia-settings I get the ‘no kscreen background’. This was a bit counter-intuitive, but I tried it as it was suggested in this thread. It is now working as I like it, but I do not know if the setup will survive a reboot.