Well, I know 2 way SLI GTX 260 is about equal to a GTX 470. I also know that three way sli on those 200 series scaled HORRIBLY.
I know the GTX 580 would be an upgrade (on so many accounts, power, heat, noise, and memory amount, DX11 and even graphics horsepower), but it wouldn't be THAT much more powerful.
EDIT - http://www.tomshardware.com/reviews/geforce-core-216,2120-3.html
Actually judging by that review.. I would say its a definite upgrade, it looks like 3 way SLI 260 is about damn EQUAL to 2 way SLI 260.
Which would mean 3 way SLI 260 is about equivalent to a GTX 480 I'd say. So a GTX 580 would be an upgrade, again it still wouldn't be a HUGE leap.