A regulation that penalizes a player who gets involved in a fight that is already in progress. This player is called the "third man in" and receives a game misconduct penalty so that he is not permitted to play for the remainder of the game. It is up to the referee to judge whether or not a player was the third man in an altercation. The Third Man In rule should only be applied when a fight occurs, and not when players are pushing and shoving one another during a stoppage in play.
The only situation when a player can intervene in a fight already in progress without being subject to the consequences of the Third Man In rule is if a match penalty is handed out in the original altercation. For example, if a player punches an unsuspecting opponent and injures them, the third man in is allowed to enter the fight in defense of their teammate. The player who threw the sucker punch will normally receive a match penalty for their actions so the third man in will not be penalized under the Third Man In rule.