Multimonitor support on Linux exists, but it's nothing resembling user friendly—getting it working will require a deep dive into the internals of your window manager.
You have a few options, in order of increasing difficulty:
- Configure your OS to treat all 3 monitors as a single, gigantic monitor. This is relatively easy, but obviously quite limiting.
- Use command line arguments to force X-Plane to allow you to configure three monitors. (Instructions on this below.) This may still require you to fiddle with your window manager (possibly extensively!) in order to get the actual window positioning right. (The OS doesn't provide a way for us to position windows such that the window manager won't move them... so it has to be done manually.)
- Figure out the exact, perfect xrandr configuration that allows X-Plane to configure the three monitors directly. This is possible—I'm told some people have figured it out—but I can't offer much guidance. The discussion here may help.
As you may have guessed, I think #2 is your best option (assuming configuring the displays as a single, triple-wide display won't suffice). By launching X-Plane with the command line argument --monitor_bounds, you can specify the size and position of each monitor manually, like this:
--monitor_bounds=<left>,<top>,<width>,<height>[<left>,<top>,<width>,<height>, . . .]
So, for two 1080p monitors side-by-side, you'd do this:
For three monitors, you'd obviously add another set of 4.
For a bit more discussion on how we came to this state, see the comments from Ben Supnik (X-Plane Desktop's product manager) here.