As you can see a gtx 580 in sli uses under 800w and that's 100 percent loaded in furmark, In games you will never come close to that. I think a good quality 850w psu will be fine for two cards and go for the 1000w for three.
1000w for three ??? realyy
Ok what do you recommend for three cards?
You do know that total system watts for two 570's will be around 500w to 650w or so right? 1000w is actually more than enough for three 570's.