Hi there, welcome
You’ll want to wait until Osito is neutered until you go through any introductions and pre-bonding. Hormonal boys are VERY stinky and tend to upset other buns, especially bonded pairs.
I don’t think it’s hopeless, because Riley’s behavior towards him will certainly change once he’s neutered.
So for now, keep them separate, and not housed next to each other. Also keep an eye on Riley and Momo to make sure their bond hasn’t been upset. If this does happen, it doesn’t mean the three can’t be bonded, it just means you will need to keep them all separate and then work on bonding them as a trio.
Sirius and Luna went through this with her 3, and now has a happily bonded trio.
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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.