Tom's Hardware Taiwan has scored some early three-way SLI benchmarks and they've done so with the help of MSI. The motherboard used was the upcoming P7N Platinum and Tom's Hardware ran a setup of three 8800 GTX cards in the board. They also had a play with three 8800 Ultra's but it seems like the rigid SLI bridge that connected the cards didn't fit that well with the Ultra cards judging by the pictures.
None the less, it seems like three-way SLI has a real advantage, at least for those that have high-resolution displays, i.e. above 1,600x1,200 and more like 1,920x1,200 or higher. Tom's tested the setup with 3DMark06, Prey and Splinter Cell Chaos Theory and it seems like the biggest improvements was seen in the Splinter Cell.
However, with double the performance if not more in most cases, it looks like three-way SLI might be a sound choice for those looking at playing games on their super-high resolution displays. 3DMark06 started out at gaining a mere 1,500 points at 1,280x1,024, but when you hit 2,560x1,600 then it really takes off with the single card benchmarks sitting at 4,752 3DMarks while the three-way SLI setup scores 10,145.
The numbers are even better in some of the games with the figures in Splinter Cell being about 2.4-2.7 times as fast as with a single card depending on resolution, although even at 1,600x1,200 you're only seeing about 1.8 times performance increase. This means that there's no point even considering a three-way SLI setup unless you've got a screen to play on, since you're not going to see the real benefits of it otherwise.
You can find the full article in Chinese here, although there are a few pictures and some more benchmark figures you can check out.