Odyssey
Riddle: Alone I am 24th. With a friend, I am 20. Another friend, and I am unclean. What am I?
Answer: The letter X. It is the 24th letter of the alphabet, XX in Roman numerals is 20, and XXX is a label for movies that are inappropriate (unclean).
Source: https://www.riddles.com/index.php/5876
