Drawn from a group of English soliders calling themselves the Three Musketeers, whose motto was "All for one and one for all".
1. If caught engaging in questionable/illegal activities in a group, one person takes all the blame (one for all), and the others who were involved do their best to help him out with whatever consequences he may recieve (all for one).
2. The understanding that if everyone in a group of people breaking the law/doing things they're not supposed to gets caught, they will all claim to be the sole person responsable. (Each one person for all) This way, either none are punished because of the confusion, or all are punished equally. (All for one)
Hey, if we get caught smokin' this ganja, who's gonna be three musketeering?
Okay, if something goes wrong, we're three musketeering it. Everyone says it was all them.