Monitor setup: http://img11.imageshack.us/img11/4044/r7zo.jpg
Game fullscreen windowed: http://img547.imageshack.us/img547/8014/ixh.jpg
I've made a simple program that removes the window border and fits it onto the primary screen.
Go into video options, set it to Windowed (not fullscreen windowed), then change the resolution to the same as your primary monitor, and run the program. It'll fit the window to the primary monitor. You have to run it every time you start the game.
Download: http://treesoft.dk/arma3/downloads/ARMA3BetaMultiMonitorFixer.zip
https://www.virustotal.com/en/file/ce12ba9a00dee26b70a493576d17451ed288dbce88f637a71ec1eb10ad886ad2/analysis/1371830818/