The answer to the riddle of the sphinx is MAN, which is a three letter word. The riddle of the sphinx is an ancient Greek mythological story about a sphinx, a creature with the body of a lion and the head of a woman, who posed a riddle to travelers passing by. If they could not answer the riddle correctly, the sphinx would devour them. The riddle was 'What walks on four legs in the morning, two legs in the afternoon, and three legs in the evening?' The answer to the riddle is MAN, as a baby crawls on all fours, an adult walks on two legs, and an elderly person uses a cane for support.