Threeway SLI is still a hassle, talk to anyone whose done it and they'll agree. It's one of the reasons why NVIDIA officially announced that they didn't support it with the 1000 series. A single more powerful GPU is almost always better than two lesser GPUs.
You may be able to get away with 480's in two way SLI with better performance without many issues, but ultimately my earlier statement stands - always choose a more powerful GPU over SLI.
However, the 480 has yet to be tried and tested, so everything we know about SLI scaling could be disproved (though it's doubtful).
More about SLI Scaling...
[video="https://www.youtube.com/watch?v=UnS0xWtoRzk"][/video]