Try turning everything as low as possible and if its still really LOW fps then I have no idea, turn it all off and work your way up the settings.
Its a weird engine that is not well optimised at all like the person above me said. For instance, the FPS difference between having shadows on LOW or HIGH is negligable, so you may as well put it on high or off as the only settings worth using.
Same with AA, and the memory usage thing is pretty weird default uses all your cards memory but Very High only uses 512mb.
I got the game running OK, it looks nice and runs at around 20fps, sometimes more depending on where I am in the world.
All we can do is hope that ARMA3 engine will be more of a massive fix for the ARMA2 engine rather than a whole new thing with the same crap optimisation.