on my test rig it has a hx750 psu with 3 7950s puling 700w from the wall has been going for nearl 3 monts psu output temp is only 40c

I'm running four with a Corsair 1200AX, but you could get away with less. I'd recommend the Corsair AX860i; it's 860 watts and super efficient. Mine is powering two 7950s and never draws more than 415 watts... should be plenty of headroom for three 7970s, especially if you undervolt them a bit.