More $0.02 for you:
I am under the assumption that you have two adult gliders and they have bred. Now you are about to have three. I do not recommend handling these joeys until they are 7 weeks out of the pouch. At that point, you should pull him/her from the mother and introduce it to a fruit diet and commercial diet. It is also crucial that you make sure that the joey learns to drink from a bottle versus a bowl. This is a difficult situation. I feel that a glider should never be alone. In saying that, you should get a joey is about eight weeks old that is already weened off of its mother and let them be together by themselves. If you introduce another joey into the same cage that the adults are in, there is a good chance that the original mother may hurt or kill the joey that isn't her's. I would not consider myself an expert at all. This may be a question for the Administrator of this forum.
Another note, I do not recommend breeding gliders unless you plan on not handling them very much anymore. Gliders tend to get very aggressive once they begin to breed. Also, due to USDA regulations, you may not sell, trade, or give the gliders away unless you are USDA licensed.
I hope this helps. Good luck!!