Yeah, GPUs in SLI and Crossfire don't share RAM as far as I know. Basically, one GPU renders one frame, then the other renders the next frame and so on... and each GPU requires it's own memory to do it. I know if you can run Crossfire between a 1GB card and a 2GB one, but if you can it would basically be like having 1GB per GPU, the extra GB on the second card would not be used.
In my experience, I have NEVER been able to "MAX out" game settings in Eyefinity.
You're running 3 30 inch displays? Given the benchmarks results I've seen for 5760X1200 I'm guessing your frame rates at native resolution aren't exactly great.