is the carb a Keihin? I haven't come across one with 3 nipple connections but you may have a Camino?
the one on the bottom of the bowl is a combination drain and overflow. Drain is by means of the screw on the bowl. Overflow by means of the long thin vertical tube inside the bowl. On the Hobbit it's tube snakes down to the left of the bike and goes into the center-stand spring .. at least thats the way mine is routed.
As for the third nipple all i can imagine is it's a float bowl vent and the vent tube's end would go to some protected area, preventing water and dirt from getting into the bowl.