Depends on your screen size.
If you're using a triple monitor setup, go for three cards (it's recommended).
If it's one large screen, or one full HD or smaller screen, go for the 2 580's. No point of three here, except that they'll be much less micro-stuttering on a Crossfire setup. High end SLI configs have less stuttering than their corresponding xfire setups.
Check out the GPU charts on tom's, read that micro-stuttering article and check out SLI and Xfire scaling as well.