I would recommend doing some prebonding, which is normally swapping them between cages, but if the pair don’t have a cage, you can swap litter trays, food bowls, blankets and toys. It’s also important that they can’t touch through the cages, as they could easily bite and box through the bars. I recommend a double layer of bars around the caged bunny. Swap litter trays etc for at least a month, before moving on to actual bonding in a neutral space.
Here’s a post I made recently on someone who was bonding a trio, which is also semi relevant to you:
I recently bonded a trio!
The main thing you need to be aware of is the risk of splitting your already bonded pair. Often, the smell of a new bunny can cause a bonded pair to start fighting, or for one to start bullying the other. You need to keep a very close eye on your pair and be prepared to split them up if you see any worrying behaviour. It is entirely possible that you could end up with three separate bunnies who refuse to bond, but not necessarily likely.
I had to split up my existing pair, as Luna started bullying Atlas when she could smell Nimbus. This meant it was easier to prebond, as I could swap them all between cages without worrying about aggression.
Personally, I started off with the pairing I knew would be easy, the two boys, then once I was confident that I wouldn’t have any problems between them, I worked on the trio as a whole. That way I knew if a fight broke out, I had to focus on removing Luna. Luckily, as I monitored extremely closely the whole time, we didn’t have any fights.
You need to think about the personalities of your rabbits, and think about what will work best for them when bonding. Rushing bonding is the main reason that it fails, and its especially important to be patient when you have more bunnies and are risking splitting a pair.