Well, I've never bred or had intention to do so, but I do have a trio. I have my first pair, both females that I homed at around a year old, about two months age difference between them. They were from the same breeder
and their adoptive family had them for almost a year before rehoming them. I had them well over a year when I homed a four year old neutered, and recently single male (his cage mate, a much older female had passed). I went ahead and did the 30 day quarantine and two fecal tests, even though I was sure he was okay. After the quarantine and second fecal was alright, I began the standard introduction procedure of scent swapping/mingling. I wasn't able to place their cages near each other or even in the same room, I also didn't do cage swap. I just used fleece blankies for mingling their scents and carried them in separate, single layered bonding pouches
at the same time. After some time of doing that, I faced the screened windows face to face. They looked at each other and they were all quiet. After a few bonding
visits like that, I introduced them, it took two tries, but done the same day. They have been together ever since, but now sometimes my boy has some "bully" issues, usually related to food. It's not serious, just fussing and chasing, but it sounds
mean and I don't like my girls dealing with that. Apparently though, it's just the way of gliders and it happens sometimes, but not all the time. I'm lucky I guess that it hasn't escalated so far or hasn't been worse as I've read it can be in some cases. I'm just perplexed because they were like the three amigos for the first several months.
I know I didn't answer all your questions, but wanted to share my experience of having a trio and how it came about. With young joeys, you may find things are easier, I don't really know.
Also, my male is neutered and not being so may change things.
Do you mind me asking why you are breeding? I'm curious if you just want to experience it, or create a family colony, or if you want to sell.