This one should cover all your bases on doing a 3 way SLI config.
Lifetime warranty is also a good bonus as well.
I use the 750WLSP and will be upgrading to this very one myself (Mid-Tower + Mountain of cables = Messy as hell case).
Ultra X3 ULT40071 800-Watt Power Supply - ATX, SATA-Ready, PCI-E Ready, Energy Efficient, Modular in Canada at TigerDirect.ca