Ummm unfortunately not. SLI is actually only capable of *2* screens on Vista with the latest drivers, other than that it does not support multi-screens. Your better off with one monitor and 2 cards, the third card will only work on a 3XSLI motherboard, and it would have to be working together on the same monitor with the other two.
I think you should call nVidia support and ask them. You are talking about buying a $200+ 780i MB and three expensive gaming cards from them after all, so I'm sure they'll be very motivated to help you
Edit: btw, if you really want that scenario and nVidia can't do it, call AMD's support too and see if they can do it with a P5Q-E or some X58 motherboard or a K9A2 Platinum and three ATI cards.
Open the Device Manager, and under Display Adapters you will see all three cards listed. Right-Click on the third card's entry and choose the "Disable" option. Close the Device Manager and reboot the system.
Once Vista fires back up with the third card disabled, the option to run SLI will show up in the NVIDIA Control Panel. Enable SLI as you normally would, and regardless of whether or not you have to reboot to enter SLI, once SLI is enabled you must reboot at least one time so that Vista starts up with SLI active.
Once SLI is enabled and Vista has been rebooted, you can open the Device Manager again and re-enable the third card.
At that point, you can do whatever you need with the other monitor(s) as you normally would. You can't extend the SLI rendering to the extra screens, but you can still extend the Vista Desktop, etc. and run apps or child windows as you normally would.
To get out of SLI, reverse the process. The key is in having the third card DISABLED while enabling or disabling SLI with a reboot.