Interesting...I've been running 3xHD (5900x1080) for 2½ years and the majority of my hobby builds are 3xHD. So telling me about what can & cannot be done is like talking to the wall. I 'get' vRAM bottlenecks, scaling, rendering quality, 3D Vision and yes even AMD HD3D not to mention differences with LightBoost.
Specifically to BF3 the frame rates I mentioned are max settings with 4xAA and 16xAF. The minimum I can tolerate is 2xAA otherwise it's like watching an Xbox.
Personally, I'd follow my recommendations with a MOBO with an Ivy Bridge or if a SB-E then my picks are either the ASUS R4E or R4F. You'll have room to grow vs being stuck & SOL.
Here's the problem, TODAY with 3xHD you're barely making it work, but newer and more demanding games you'll be hitting a wall with nowhere to go.