Hi there
I don’t have any personal experience with trios, but I know the risk is always that trying to bond the trio breaks up the current pair. That said, since you have space to accommodate them if things don’t work out, it is less scary to try. I believe most trios are usually two males and a female, but often they start with a male female pair, and a second male is added.
You hit it on the head with your concern about their personalities. It’s pretty hard to predict this without seeing them together. So I think once he is neutered and his hormones are settled, I would plan to do a speed date with all three in a very big very neutral space. You can see how they react towards each other and decide then. Keep an eye on your girls as they interact more with the boy to make sure his presence doesn’t throw them off. Since they stayed bonded through their spays without significant issues I’m guessing they have a strong bond.
In some cases, the trio bonding process disrupts the current pair, but that doesn’t mean the trio won’t work. The dynamics in groups are always different than in pairs, so it just means you end up focusing on the trio as a whole (vs. adding 1 to the pair, if that makes sense).
Member Sirius&Luna went through this with her trio, her thread might be informative to you: https://binkybunny.com/FORUM/tabid/54/aft/165467/Default.aspx
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.