Threesome or moresome! Because why stop at two sparkly boys or girls when you can have more!
There are quite a lot of moresomes around, but most of them don't have a lot in the way of plot, being All About The Porn. So my pick is
The Other Side of the Street by Puszysty, in which Kevin and JC swap bodies (as you do) and everything ends up as a Backstreet/Nsync orgy. Well, it would, obviously. Good stuff!
My (possible) favourite from my own supply is
Let's Do the Time Warp Again, featuring Joey as Eddie, Justin as Rocky, and Lance as Columbia.
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