Nvidia still has to make an announcement about the subject, but the folks at German PC gaming hardware site, er, PC Games Hardware have actually managed to benchmark such a configuration. Their set of three GeForce 8800 Ultra graphics cards seems to yield a significant performance improvement over a standard two-way 8800 Ultra SLI setup, as well. In Crysis, for example, the three-way SLI doubles minimum frame rates from 7 FPS to 14 FPS and boosts average frame rates from 27.1 to 37.9 FPS. PC Games Hardware's tests also include Unreal Tournament 3, F.E.A.R., and Company of Heroes.
Across those four games, three-way SLI improves minimum frame rates by a factor of 2.5x and average frame rates by a factor of 2.4x over a single card, compared to respective factors of 2.1x and 1.9x for two-way SLI.