I would say PG, SF, and C would be the three you would like to have all stars at ideally.
Aa far as all time 3 man teams I would go with MJ, Wilt, and either Larry Legend or Magic. I realize that a lot of speed and lateral quickness would be very important in a game like this but not anymore so then passing and shooting. I would take Larry and Magics game over Kobe, and Lebron although I would certainly not be disappointed to have Lebron. Keep in mind though, if Wilt is playing D around the basket that Lebron is way, way, way overmatched and would get hurt trying to drive the hoop with reckless abandon as he does now. And, Wilt would outscore everyone on the court in a 3 on 3 game. Just my humble opinion.