Kokusai Sannin Shogi (International three-person chess) is a modern variant of Shogi (Japanese chess) for three players, created around 1930 by Tanigasaki Jisuke.
The game takes place on a board of 127 hexagonal cells. Each player controls a set of 18 pieces. The game allows for two players to enter an alliance against the third. If two players engage in a coordinated attack against the third, they are entered into an alliance automatically. The player not in an alliance gains a more powerful king piece and easier win conditions to counterbalance the alliance. As in other games in the chess family, the goal is to checkmate an opponent's king. A player not in an alliance need only checkmate one player to defeat both of them.
Unlike traditional games in the chess family, which have a thin military theme, kokusai sannin shogi has a thin political one, with pieces named after concepts such as public opinion, diplomacy and propaganda.
Incorporates information from: http://en.wikipedia.org/wiki/Sannin_shogi