If Nvidia Surround it what you are going for, than like mentioned by others, you will need a second GTX 580. But if you are just wanting 3 active monitors, you could also just get a less expensive card for dedicated PhysX, and also use that for your #2 & 3 monitors.
I know many will shun the idea of a dedicated card for PhysX, but
in my testing using two (2) GTX 580s for SLI and a 9800GT for PhysX, games that support it get a higher FPS vs just using the 580s alone.
But the big advantage for the PhysX card, is that your #2 & 3 panels will be active even when in-game with SLI enabled. When I have all 3 panels hooked up to my 580's, as soon as I load up a game and SLI triggers, only my primary display stays active, #2 & 3 go blank. I like to monitor my temps and sometimes have a TV show going on my secondary monitors while gaming in SLI, so the PhysX combo works great for that.
Here is an example of what I have going. My primary 24in display is hooked up to my #1 GTX 580, the other two panels are connected to my PhysX card (9800 GT). I have them in portrait because I eventually will get a 30in for my main display, and they will be the same height & vertical resolution as the 30in when set up that way. I hope I was able to help in some way.
