OCWorkbench has some more information about NVIDIA's three-way SLI. The site says it's currently only working under Windows Vista and that the performance boost can be as high as 2.7x:
With Forceware 167.10, 3 cards can be recognised. In nTune, there is already the option to turn on 3 way SLI. When executing 3Dmark05/06, a mark showing SLI x 3 will appear on the screen. At this moment, the driver are not optimised, thus performance levels might appear to even lower than 2 cards running in SLI. It is said that Forceware 167.25 and above should resolve the performance issue.
According to NVIDIA, with 3 pieces of 8800GTX paired with Core 2 Duo X6800 with nForce 680i chipset, performance of 3 way SLI versus single card is 2.6 time for Splinter Cell: Chaos theory (2560x1600). As for 3Dmark06, the performance increase is 2.1x.