Object: Checkmate the opponent's King or check the opponent's King three times. Other rules are the same as in standard Chess.
 
The game is more tactical then standard chess, sacrifices are very common, there are a lot of traps in the opening. Two checks is already a big advantage and may worth more then a piece.
 
Three Checks Chess is mentioned in Pritchard's book 'Popular Chess Variants'. Karpov is said to excel in this chess variant.
The game can be played online on www.brainking.com. Also you can view a lot of sample games there.