UPDATE!
I found a power supply for $329 that a pro miner who would know assures me will handle 3x 7990:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153154He says even a 1200 watt would probably work. However, I'd err on the side of caution/overcapacity, since you are less likely to make some noob mistake and fry something if you have an adequate power supply, and also I have read somewhere that PSUs operate at maximum efficiency when putting out 50% of the power they are capable of putting out, and efficiency above 80% starts going way down because of heat reducing efficiency. (as a conductor like copper heats up, its ability to conduct electricity goes down)
Also, I've been told by this same pro that I shouldn't "piggyback" power supplies on top of other power supplies, but I do it anyway! 8D You can do it with a paperclip or one of these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16812201037