Well considering all of the PSU's that I have seen to be tested and certified for three way SLI are at least 1100 watts with the average being 1200 watts.
So I would say that you need at least an 1100 watt power supply.
Will three high end video cards really make things look any better?
After doing more research there is one 800 watt certified power supply, but here is the official website with all of the approved power supplies.