The position on the board must be the same with the same player on the move for that player to claim the draw. These three identical positions do NOT have to be 1-2-3 in a row, but can be seperated by other moves as long as you come back to the core position for the third time. A few notes:
1 As mentioned, the status of both Kings for castling must be the same and all en passent opportunities must be the same.
2 Any pawn move resets the counter, since they can't move backwards.
3 The draw must be claimed, see the "claim draw" button here at RHP interface.
4 The player claiming the draw must make a move that brings up the same board position for at least the third time. It can also be the fourth, fifth, sixth time, etc if you didn't see it or want it on the third time.
It doesn't come up that often, but I have saved 4-5 games from lose with this method.