I'm back. This is a little hard for me now, because of my recent loss of Lulu.
I had "homed" a pair of female gliders I found on CL. The pair had originated from a local MD breeder (Glider Invasion) I was told. Lulu was the first adopted, then two months later (about) they adopted Skadoosh. They lived together in perfect harmony the whole time, from what I can tell before I homed them, and the entire time I had them both with me (until loosing my Lulu). Skadoosh was the more laid-back glider that seemed much more social and friendly, while Lulu was a hot mess. I used to refer to her has spawn of satan. Anyway, that's a different story that did have a happy ending, after I realized she was fear-driven.
So, here I was with two girls, our relationship was good, they got along with each other, and with me. But, I started contemplating getting more. Like you, I'd consider it, then brush the idea off with thinking I didn't want to mess with the harmony we had going on.
Eventually though, over a year later, I saw Ollie (named Icarus) in need of a new home on a glider forum (not this one). He was fairly local, he was a former rescue, recently single (cage mate passed), and aprox 2 yrs older than my girls. His then mom was a good one, and had provided at least this little guys 3rd home, so I knew if I got him, he'd have to be a keeper whether he meshed with the girls or not.
I made the decision to home him.
I didn't have second thoughts with this one after I made my decision, like I had originally with the girls (cold feet), which is from what I've read very normal.
So the protocols of quarantine (probably wasn't necessary in this case, but I followed it anyway), and introduction prep following Q.
It took 2 attempts, finally I had a trio. This is another long story.
I haven't regretted having a trio. Even with some food aggression, that was fairly mild anyway, being just fussing and chasing and not even all the time. Sometimes I'd see two, or even all three of them eating together fine. They all slept together, and it seemed they were happy together. I never noticed a change in my girls attitudes towards each other, or me that was caused by Ollie. I did have a little change of attitudes towards me, but that was my fault, nothing to do with having a third glider. I'd gone through some long periods where I couldn't be consistent with their out of cage play or pouch visits. But again, that would be another long story. My personal experience, I felt I made the right decision, also when Lulu passed, it gave Skadoosh a cage mate left. I didn't think of it in this way before, no one wants to, just saying it helped in my case. I'm not in a position to get another glider to introduce at this time. When the time comes, and I hope it's not soon, I don't even know that I'd get anymore. If I did, I'd look for at least pairs already together.
So to answer your question
Yes, it could be cold feet, and understandably natural for you to wonder if this harmony would be spoiled. I can't guarantee it won't, but it didn't in my glider's case. There may be some disruption if they all combine: reestablishing dominance, food aggression among the top of the list. But if they all accept each other, and don't become overly violent, they'll be fine, and maybe even happier.
Note, If you find there is some issues with introductions. Give it some time in between re-tries, based on the severity. Also, you could try pairing the new guy with the gal that gets along and let the angry one have her own cage for a while, this sometimes checks their behavior (from what I've read).
This doesn't all guarantee you'll have a successful trio, so what you need to ask yourself to make your decision is..Can you be prepared to keep two cages?