It hasn't always been three either. Once upon a time it was five.
In some other games like Stratego or go repetition is forbidden: you are not allowed to repeat and you lose if you cannot make another move.
In chess and some other abstract strategy games, the threefold repetition rule (also known as repetition of position) states that a player can claim a draw if the same position occurs three times, or will occur after their next move, with the same player to move. The repeated positions do not need to occur in succession. The reasoning behind the rule is that if the position occurs three times, no real progress is being made and the game could hypothetically continue indefinitely.
In chess, in order for a position to be considered the same, each player must have the same set of legal moves each time, including the possible rights to castle and capture en passant. Positions are considered the same if the same type of piece is on a given square. For example, if a player has two knights and the knights are on the same squares, it does not matter if the positions of the two knights have been exchanged. The game is not automatically drawn if a position occurs for the third time – one of the players, on their turn, must claim the draw with the arbiter.
Becuase repeating until 50 move rule kicks in is boring and pointless.
You can't post in the forums yet. Play some games!