condog209

Ok but changing things like the cap can change variables so try with uncapped too.

Then the 480gb. i guess. What brand are you insisting on only using?

AMD have historically allowed Crossfire-ing of different cards from the same series. Since the R7 and R9 GPUs are just rebranded (excluding the R9 290, 290X and 260 I believe) they will work in Crossfire with most cards from the 7000 series.
 
I wouldn't really recommend this unless you already have one or more of the cards as the improvements may not be great. As a rule of thumb, get the highest single card you can before moving to Crossfire when considering a new set of cards.

It's not worth it if you're thinking of buying all three. I'll tell you why. If you crossfire a 7970 with a 7950, the 7970 will become a 7950 effectively. Same goes with the 280X with the 7950. The 280X will effectively become a 7950.