"Тройбан" is a slang word for "три" (a ruder version of 'three'). There is also a word "трояк" (less rude and more common). The affix "як" can be attached to some other numerals to make it more informal, e.g. "двояк", "пятак".
But the affix "бан" can be found only in the word "тройбан". So, I guess, this word has a special, particular origin. I heard a story that the word "тройбан" is derived from "троебан", which, in it's turn, goes from the phrase "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?