"For going multi-graphics cards, you need to ascertain these things,
Does your motherboard support 3 graphics cards?
I think each would be x16 given your question & PSU detail, I guess you have a good SLI capable board SLI PSU ref
Will your power supply unit take the three cards?
this is in terms of wattage and connectors I get the feeling that @1200W, you probably have the connectors too, but worth checking
Once you get these things sorted out, the next important angle is the thermal one
Do you think you have enough cooling and circulation to handle three heat producing elements kept close to each other?
Do you have a multi-monitor setup? or, a high resolution (beyond 1600 horizontal) display?
Most of the configurations are not worth unless you have all that visual real estate look at the SLI FAQ on multi monitor setup to figure out your display setup
Will you be utilizing all this effort with the applications you use?
Are the games you play capable of using all the hardware that you plan (look at the game specs and reviews)
I put up some references in an earlier answer to Is it possible to run two separate video cards? yesterday.
Update: 3-Way SLI Gets PhysX Debut with Cryostasis and references within.
Legit Bottom Line: A dedicated NVIDIA PhysX card improves performance across the board and a pair of GeForce GTX 280 video cards in SLI with a dedicated PhysX card performs better than a set of GeForce GTX 280 in 3-way SLI!"
"What resolution are you running at? From my experience two 280's will be able to run just about anything on a 30 inch monitor at 2560x1600. Getting a third one would really be overkill if you aren't noticing a problem running some games.
From your setup it looks like you've got a killer rig. If you're thinking of buying high end cards right now I'd say wait until the next flagship cards get released. NVIDIA should have one out towards the end of 2009 or early 2010 and ATI perhaps a bit sooner. Those cards will (possibly?) support DX11 also, something worth noting."
