You’ll find boatloads of anecdotal evidence about bonding multiples together, both positive and negative. A big part of it is your buns’ individual personalities. For example, I had a pair bond in 3 days, and an extremely easy, no fighting, 2 weeks trio bond. On the other hand, it took my friend THREE YEARS to bond her pair, completely with dangerous fighting, and it only worked cos we finally figured out that one had poor vision and needed its partner to wear a bell collar so as not to startle it into attacking. A third would NEVER work.
Is one of your pair have a territorial bun? A possessive one? One that might be unhappy feeling crowded or “competing” for food?
I really enjoy my trio. I think that one is definitely better off for it, one is indifferent, and one honestly, though they all get along, would have been just as happy to be a bachelor for the rest of his life. Clearly you’re thinking about it if you’re asking, please don’t think I’m implying you aren’t. But while they may eventually bond with a third– is that best for them? I think I would let them really solidify their own bond for a good long while and watch them. How bossy is the bossy one? What will adding a third do to the submissive one? Etc.