Hi and welcome.
I agree that you'll need to get them all to a really good glider vet, and happy to see that you are doing so. It was too early in your case to try to put them together. Especially with your first two being with you for such a short amount of time. They're really still adjust as well. The vet saying that some of this could be due to Mo having not "grown up" with a cage mate is odd. 7 months old is young for a glider and given the time that he needs to acclimate to all of the new things going on, he should be able to get over some of these initial behaviors. Each glider, as you've seen, has their own unique personalities. They bond in different ways and one totally different time lines. With humans and other gliders.
See what your new vet has to say. Now that all three have been introduced they'll all need fecals and if one is positive they will treat them all. Then take everything back a step and allow them time to know that there are others in the house and slowly move the cages closer...not close enough for them to reach each other. See how they react for a couple days and try the introductions again in a neutral spot...your tent or glider proofed place will work for this.
Good luck and keep us posted on their progress.