heres a nice review on that
XFX HD6970 3-way CFx & HD6950 CFx review | KitGuru
they have 2x and 3x in there. notice the minimum FPS
the difference there is very small between 2 or 3 cards
also for temps, 3x is a bit higher then just 2 cards
review with eyinfinity being used
XFX Radeon 6970 and Radeon 6950 Graphics Card Review - 3-Way CrossFire / Eyefinity vs SLI