Copied this from the FAQ:
"Third repetition of a position. If any identical position appears three times during a game, then the game can be claimed as a draw. Please note that each repetition must be identical with regards to en passant and castling availability. A draw can only be claimed along with a move that contributes to a repetition. Claiming after the repetition has occurred is not possible, unless the same position can be repeated again on a future move."
I gues it explains it to some detail. Cheers.