I'm not hard-and-fast on the matter -- in a case like this, I'm willing to have one pair "win" if that fits the casting otherwise, and cast the odd person somewhere else. (Flipping a coin really isn't an option, since all things are really never equal in this situation.)

But in the actual case at hand, there weren't any really natural pairings anyway, so the easiest thing to do was to simply cast all three into their otherwise-best fits, which wound up with all of them in separate parts of the game...