What GPU do you have? I have issues with setting it to triple screen ingame, so I set it to one across three.
Set number of screens to one in AC.
If you have nVidia got to control panel, click span displays with surround and then click bezel correction. Correct until the road image lines up. The click on resolutions in the same menu you clicked on bezel correction and look at the "in use". Pick one of the bezel corrected ones. One should be slightly higher than your native res, and one is lower.
Go to documents/assettocorsa/cfg and open video.ini. Set resolution to exactly the number you picked from the list. Done. After that you can use the +/- on numpad or the onboard settings app to adjust fov, seat position, etc.