I think you are good.
A single gtx580 is recommended to have a psu of 600w.
Each added GTX580 potentially can use 75w from the slot, 75w from the 6 pin connector, and 150w from the 8 pin connector. That totals up to a max of 1200w.
But, it is most unlikely that all of that power will be drawn, and the corsair psu should be conservatively rated. Since you own the psu already, at least see how it does.
And, ...I really doubt that a third GTX580 will be worth the upgrade. I suggest trying two first.