I couldn't use triple screen because my monitors have different sizes (23''-32''-23''). But now it is working, and without SoftTH and without Eyefinity. I just had to change the resolution to 5760x1080 in video.cfg (documents/Assetto Corsa folder), unmark fullscreen, mark triple screen and change, in windows, monitors resolution to 1366x768-1920x1080-1366x768.