Well, Ultra is not a high end maker of power supplies, some of them were simply awesome units, and some, well they simply were dangerous to load anywhere close to it "ratings"
dual 6970 can happily feed 2560x1440 resolution, gaming, in most games the Radeons do not scale well past the 2nd card for anything but the dual X2 style cards(quad fire) or the 7900 series, its really not worth the added cost, heat, power consumption as alot of games show very little playable difference.
on a 60Hz monitor, if you got say 1000FPS it wouldnt do any better then it running 100FPS, so, unless the games or watever that you are using are tapping out of what the 6970s can give you(I doubt) there is no direct reason for tri-fire of them. Now if you were bitcoin mining, folding, ripping movies and such that can use the extra gpu, that would be different.
Save up the coin, and go for dual 7950s if you want the extra performance