"Pairing": two's productive, three's a crowd

I find it productive and rewarding to have two people sitting at a computer. But if three people sit at a computer, things don't go so well. I get the feeling that we usually get less done than with two people, which is frustrating as well as wasteful.
So if somebody comes and joins your pair (probably because you wanted to show or ask them something) I suggest that one person should duck out. I call this "bump pairing" because somebody gets bumped out by the newcomer.

What should you do after being bumped out? You might well join somebody who is "odd" (sitting by themselves). It could be the person left behind by the newcomer who bumped you out: a straight swap. Or you could fill in for somebody missing through a natural break like having a smoke or taking an important call. A different option is to offer help to a complete pair. If your help is wanted then logically you should bump out one of its members. And so another person is free...

Bump pairing could result in faster pair rotation. I think of it as providing a higher frequency wave of pair rotation, overlaid on a more traditional slower wave of rotation. In this model each member of the team belongs to a single natural pair (the one that signed up to deliver a story, for example), but might also find themselves working in a number of short-term temporary pairs. I suspect something like this happens already in most teams, but it is useful to explicitly recommend that trioing is avoided.