It seems that aspect ratio is not updated when changing resolution. When I disabled Nvidia Surround graphics were still messed. But then I noticed that aspect ratio was set to custom. I changed that to right one and now graphics seem to work on single monitor. So fix might be as easy as just updating aspect ratio when changing resolution.

Default in Steam for Screenshot is F12 (Hope that helps)

I can confirm this and uploaded a pic of the main menu.
My FOV Settings:
fovTop=0.75;
fovLeft=4;
tripleHead=1;

And Resolution:
fullScreenWidth=5760;
fullScreenHeight=1080;

This settings works great with NVidia Sourround in ArmA2:OA. In A3 those will mess up the UI as seen in the picture.

I experience a similar problem.

My computer has a NVidia GeForce GTX 650Ti.
My Monitor has a native Resolution of 1920x1080 (16:9)
This was detected correctly (had to switch the aspect ratio from 16:10 to 16:9 manually afterwards).
What i also noticed is, that the resolution did not work in the game, i only had a real 3D resolution of about 1280x1024 if not less (i could only guess, however it was horrible low, like in the 90s).

It looked like if the engine tried to render the high-res GUI elements into the low 3D resolution.

The problem is solvable as workaround by reducing the resolution manually to about <1400x...

Closing due to lack of activity / presumed no longer valid. If the issue is still reproducable for you, please create a new ticket requesting this one be re-opened.