The greatest of boxing is Muhammad Ali, a three-letter answer to the crossword puzzle. Ali was an American professional boxer and activist who is widely regarded as one of the most significant and celebrated sports figures of the 20th century. He is known for his inspiring quotes, his philanthropy, and his record-breaking career in the ring. He won 56 of his 61 professional fights, including 37 knockouts, and was the first boxer to win the heavyweight title three times.