I am on linux mint 17.1 cinnamon. Here is my xorg.conf: http://pastebin.com/6xCncZJ5
I can move my mouse around on the 3 monitors but can not drag the windows over the borders. Cinnamon is running in fallback mode.
Edit: I set Xinerama to "on". Now my gpu monitors work normally but the mobo monitor only shows my cursor moving, on a black background.