Wouldn't you be better off with 2x6870's than 3x5850's? I mean last I heard scaling on the third card is usually pretty pathetic and if u look at at from a mathametical perspective:
5850 is about 80% of the performance of a single 6870.
Therefore 3x .80= 2.4x vs 2x performance from dual 6870's
Negate scaling inefficiencies and optimisation issues inherent with multi-gpu setups and you'd be looking at about 2x performance on the 3 cards vs 1.85x on the 2 6870's. Not much of a difference considering that you'll be paying alot more for the 5850's.
Or if you know how to overclock, prolly better off with Dang GTX's suggestion for best performance per dollar