"Тройбан" is a slang word for "три" (more rude variation of 'three'). I know that also exists word "трояк", less rude and more common; affix "як" can be attached to some other numerals to make more informal, little bit ironical lexeme: e.g. "двояк", "пятак".
But affix "бан" can only be found in the word "тройбан". So, I guess, this word has a special, particular origin. I heard an interesting story: word "тройбан" is derived from "троебан", which, in it's turn, goes from word "трое ебан", i.e. "ебан тремя" (literally means 'had sexual intercourse with three persons at once'). But I haven't found any further information about this version.
So, my question is: what is true etymology of that word?