I agree with the two other replies, ArmA 3 really isn't optimized to get that kind of FPS on ultra despite whatever rig you have.
But you will get a stable framerate on medium most of the time, or even high, dependant on your rig.
One thing I will say is AMD's CPUs aren't renouned for having good single-core performance (I have an FX processor, too) they tend to rely on mutli-core performance and ArmA games (despite DayZ SA, with the new renderer) use single-core processing from my experience/knowledge.
So if you want to get reasonably good FPS on an ArmA game, I suggest you go to the side of Intel, as they do have good single-core performance (or atleast better than AMD's processors).