At least that's what we call it over here in Australia, the concept is when you're playing pool and you've got one player vs two players... American Doubles... A 3 player game where it goes player 1, 2, 1, 3, 1, 2, 1, 3 in turn rotation would be an interesting other dimension to the game. I think for fairness sake (and because of the extra challenge) the player who creates the game is the Single, and then two other players challenge him/her?
Also, I think the player 1 needs to start with a double drop, as if it is a four player game, or maybe the same number of territories, but with 5 armies on each (3 armies = 2 avail armies to attack, hence doubled to 5, with 4)
What do people think?