Hi Hannah, Welcome to the forum 🙂
Since you just brought new bun home last week (correct me if I’m wrong) and things are a little rough so far, I would slow down a little. First I would allow the new bun to settle in (housed as a neighbor to the current pair, but make sure they can’t nip each other through the fence) for a week or two, just to let him get used to his new home and you a bit. He just had a big life change and it is probably very stressful for him. After that, I would do some “pre-bonding”, where you swap buns cages or litter boxes for a couple of weeks. This allows them to get really used to the other buns scents. At first they might be very displeased (thumping, marking, etc.) but over time you should see them calm down. You might see their behaviors sync up (all buns eating hay at the same time, grooming at the same time) or they will lay near each other on opposite sides of the fence. You’ll want to keep an eye on your pair, as pairs can sometimes split up during trio bonding, and have a back-up housing option for if all three need to be kept separate during the bonding process. (NIC cube grids are my go-to for making temporary cages/pens).
You have tried a lot of methods in a very short time, so it’s understandable that it doesn’t seem like anything is working. You’ll want to focus on building trust among the rabbits, and patience is key. You must absolutely prevent fighting. A little nipping is OK, but stop fur pulling, boxing, fast chasing, and circling.
I think in your case, after new bun settles in and you do some pre-bonding, I would work on building positive associations and trust. In your neutral bonding space, you can work with two at a time or all three, and whenever the buns come near each other, pet them both/all A LOT and swap scents. This makes them think they are being groomed by the other rabbit. Keep everyone as calm as possible, and just keep on petting. I’ve found this REALLY helps prevent things from escalating. Eventually you will ease off the petting bit by bit and see what happens, but at first don’t be afraid to have your hands on them to keep everyone calm.
I will stop there for now. I have other suggestions about the bonding space I can give you when the time comes, but don’t want to overwhelm. 🙂 For now just focus on getting to know your new bun and making sure he settles in and is doing well. 🙂
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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.