Quoted from Nvidia's site. http://www.slizone.com/page/slizone_faq.html#s7
"How many monitors are supported when running in SLI mode?
When in multi-GPU mode, SLI currently supports one monitor. When in single-GPU mode, users have the ability to use up to four monitors using NVIDIA® nView® multi-display technology and Windows XP Dualview."
You do NOT need SLI to run 3 monitors, because SLI is not capable of that. You DO however need a second video card. Although, I always thought SLI supported 2 monitors, I don't know first hand.