I use one 5120x1440 as main and one 2560x1440 as second monitor, works fine aswell over here.
But you tried to scroll all the way to the right in the AS, right? the other two should appear right next to the first one. Maybe you could post a screenshot of your AS.
OK, I found the issue, It is still a bug, but the "can not use" is wrong.. What is happening is the further left you go on really wide total display (7395 wide in my case) width the location of the AS widgets gets further and further off from the wallpaper shown in AS. What was happening, when i placed items on the far left monitor, they were way off to the right of that monitor where there is no monitor. so it appeared as it was totally broken. You can see in the attached video link,
DJI 0131 (youtube.com)