Crossfire with more than 2 GPUs has known problems (if you look carefully, you see some funky results out of Combined Test). We've told AMD. It is a driver issue.
Even without that the scaling wouldn't be 100% unless you have a very very fast CPU. Q6600... isn't. In fact, in your case the main issue is that you are CPU limited.
The problem I always have with this argument with CPU bound, Heaven Benchmark gives 100% scaling with each GPU at 98+% all the time... so for 3DMark11 GPU tests not to scale, isn't too good... it's not as though there's CPU/GPU like one of the tests
we do... just very dissapointing as all other things scale nicely, BF3, Dirt 3... yet this benchy runs like a pile of poo with 3 cards. Hopefully that driver issue will sort out.
I do agree games like BF3 in MP mode are CPU bound, which then slows the GPU's down but still.