The Tech Report played around with a tri SLI system and reports their findings.
What can I say? The picture isn't terribly rosy. In terms of performance, what we saw out of three-way SLI was more or less what we expected: that you'll need a 2560x1600 display in order to really realize the benefits of three GPUs. Even then, in none of the games we tested did we see a case where going from two cards to three made a real difference in terms of playability. (Going from 80 FPS to 95 FPS doesn't count.) The closest we came to that may have been Crysis, but that game seems to have its own performance scaling problems—at least at the "high" and "very high" settings—that involve factors other than the graphics subsystem alone. Perhaps, in some games, turning up antialiasing levels to 8X or 16X will help tease out some of the benefits of having a third GPU. That wasn't the case when we tried it in Half-Life 2: Episode Two, however.