Body and Brain Connection

Other than the obvious, in that this is achieved by playing your first 3-player "Group Exercise" what may be worth adding is that you do not need 3 players to play through the whole contest. You actually only need three people present at the start of the contest when the game/kinect has to recognize 3 different people*.

I don't know this for a fact, but I believe that once the contest has started that nothing, other than letting it complete in its entirety, is necessary. This could be the case because it never actually needs to be able to "see" you once the contest has started.

I've heard that with some of the achievements in this game, that only Player 1 will get the achievement - so that is something you may want to consider when going for this one.

* If someone wants to verify this and put it in the comments that's fine (I'll update the solution later). But, as I already have the achievement, I'm not going to create a new profile just to check it out.