Alone I stand between the 23rd and the 25th,
together with a friend I am 20,
and if another friend joins in, we will be dirty.
What am I?
I think it's
The letter X
Because
X is the 24th letter of the alphabet
XX means 20 in Roman numerals
The only thing I'm not 100% sure of is the "dirty" part
I guess XXX can bring to mind... activities which may not be exactly clean?