AFAIK, you can only run up to Tri-Sli 295's. If you have a well ventilated case and a decent WC loop, I'd go for the 295's.
The 295 is a dual card.
So there are 2 possibilities.
-1 card for 2 gpu's
-2 cards for 4 gpu's
3 cards would be 6 gpu's and wouldn't work on anything besides linux and folding.
You cannot achieve a odd number a GPU's with a dual card lol