What would happen if they all fall out then? Could you accommodate 3 single buns? And then find friends for them individually?
Perhaps a rescue could help out? A neutral area where neither of them have been and start from scratch. I wouldn't consider another bun unless they were being separated and having their own husbun. A male won't necessarily be the dominant one

In my male/female pair, it's the female who's boss!
Rabbits won't listen to you so trying to be dominant won't work

Have you tried taking them on a car ride in their carrier? Don't know if that will help but that's what people do when bonding.
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