And before I forget, if you happen run triple screen at mismatched resolutions ( like I do - 1 x 1920x1080 + 2 x 1280x1024 ), AC will support them.
Unofficially at least anyway. It's like having SoftTH built in.
The trick to get it working is to make sure that full screen rendering is 'Off'.
AC renders the windowed triple screen view centered around the middle screen, so all I lose is the view beyond the 1280 res of my side monitors.
---Steps to get it working---
P.S, ignore what the in-game res is shown as in the dropdown menu from now on, as it only shows available system resolutions.
First, make sure full screen rendering is
OFF
And that rendering mode is set to
Triple Screen
Next, manually set a resolution in video.ini ( found in \Documents\Assetto Corsa\cfg\ ) that will fit across your 3 screens properly.
In my case this was 4480(1280+1920+1280)x1080
This is so you can access the apps menu when in-game for now.
Once that's done, go in-game & move the mouse to the right edge to bring up the apps menu
Open the triple screen config app, & any other you want displayed.
Move them to your middle screen for now.
You need to do this because you won't be able to access the apps menu once you change up to a larger res.
This is because the right edge of the 'window' the game is rendering to will be beyond the physical edge of your right side monitor.
Exit out of AC & open video.ini again.
Change the res to suit, in my case it's 5760x1080 as my centre monitor is 1920x1080, and go back in-game.
The apps you had opened will still be visible/working & you can move them to the side screens now.
Plus you can tweak the side screen rotation, bevel size, etc... since you opened the triple screen tweaker before you changed res.
Might not be ideal, but at least it works