I have random crashes to desktop too with ARMA3 has stopped responding, but the game runs really nicely otherwise. It seems more common in multiplayer but it did the same on infantry showcase today too. hard to diagnose since it's completely random as far as I can tell. I know it's in alpha but was hoping there'd be some workaround like lowering graphics settings but I haven't found anything that works yet.
My system is a i5 3750k @ 4.4GHz, GTX570, 8GB DDR3 @ 1600 and game is running from Samsung 830SSD. Most other games seem very stable so I don't think it's my rig.