Alone I am 24th, with a friend I am 20. Another friend and I am unclean.
What am I?
The answer is
X
Explanation
Alone I am 24th
'X' is the 24th letter in the alphabets
with a friend I am 20
'X' with another 'X' gives 'XX', which is equal to 20 in roman numeral system.
Another friend and I am unclean
I think it's referring to 'XXX', which most people consider as dirty.