There is never a guarantee that glider introductions will be easy or that the pair will accept your guy as a member of their colony.
That said, after neutering give the new guy a month or so for his hormone levels to drop before attempting to do pouch and toy swapping in preparation for the introductions. Take things slowly. It may take more than one try to get the to accept each other. It might be necessary to introduce your guy to one of the other two first, then go through the process again to make the trio. If the female accepts your guy - her current companion will be more likely to accept him after his scent is mixed with that of the female.
You may also want to hold off on introductions for a few months to make 100% sure that there are no joeys already on the way since the female in the pair may already be carrying joeys.
If you really do not want to deal with joeys, then you are taking a risk getting the pair before the male is neutered and some time has passed to make sure you are adopting a FAMILY not just two 9 month old gliders.