This game was invented by Adam Sobey, for the Haslemere Chess Club Christmas meeting, in the 1980s. The game is regularly played in NOST and AISE, and was used in the 2nd Heterodox Chess Olympics (a large orthodox chess variant match between teams of different countries.)
The usual rules of chess are followed, but with the following exceptions:
The first turn, both white and black move one pawn. The second turn, white moves a piece (not a pawn and not the king), and a pawn, and then black moves a piece (not a pawn and not the king) and a pawn. All later turns of a player consist of three separate moves: a move of a pawn, a move of a piece that is not a pawn or a king, and a move with the king.
A player that is unable to complete his turn, i.e., who cannot make a pawn-move, a king-move, or a move with a piece that is not a pawn or king, loses the game. Also, a player that is checkmated loses the game.
The order of the three moves in a turn is not important. Castling is considered a move with the king (note that long castling is impossible, because there isn't enough time to move the three pieces out of the way.) En-passant capture may be done at any time of the move, and also when the captured pawn didn't move last in the previous turn of the opponent. When a player is in check, he only has to make sure that he isn't in check anymore at the end of the turn. (E.g., a player is only mated when he cannot lift the check with the entire three-part move.) Also, a player may move into check as long as he isn't in check anymore at the end of his turn (so, a player could move a pinned piece and then his king). (The general idea is: the three moves count as one, and only the position after the three moves is of importance.)
Pritchard's Encyclopedia of Chess Variants, and Variant Chess, issue 20, summer 1996. In this issue of Variant Chess, one also can find a few interesting game scores for this game.