Alone I am 24th, with a friend I am 20. Another friend and I am unclean. What am I?

Why this works

This riddle plays cleverly with the letter "X" and its various meanings. First, "alone I am 24th" refers to "X" being the 24th letter of the English alphabet. When paired with another "X," it becomes "XX," which represents the number 20 in Roman numerals. Finally, when you see "XXX," it refers to a common rating for adult content, linking back to the idea of being "unclean." The riddle's charm lies in using the same letter to convey different concepts through numbers and cultural references!