As I was facing formatting as Arma3 would not start after I moved hard drive I thought I would post the solution.
Basically if you run DXdiag and on the display tab it says DDI version is 9EX (should be 11) you likely have a 3rd party driver on your system that is disrupting DX11 and Arma3 won't start.
Also Catalyst display manager was showing 13 monitors available for config when right clicked which was odd (after fix shows 3).
Solution:
Go down add / remove programs in windows and remove anything remotely connected with video card installed by non ATI / Nvidia (but don't uninstall your main video driver). Mine was SMSC core drivers or something like that (Samsumg monitor driver related I think) and other posters on some sites saying other useless bloatware that came with their video card (genius!)
May also want to uninstall any monitor drivers in windows devices as may be related.
Anyway once I uninstalled that single item in add / remove programs Arma3 is fine and no longer looking at installing drivers or Windows7....
