MR is an acronym for the classic board game 'CLUE' or 'Cluedo'. The game was first released in 1949 and has been a popular game ever since. The objective of the game is to solve a murder mystery by gathering clues from the other players. The three letter solution for MR is 'REV', which stands for Reverend. Reverend is a title given to a person who is ordained as a Christian minister, priest, or pastor.