I’ve never bonded a trio so someone else can chip in there but I wanted to warn you that adding a third bun can often break up an existing bond between the original pair so you must be prepared for that to happen.

Sarahthegemini is right. Any time you add another rabbit to a bonded pair, you have to be fully prepared for four possibilities:
> RabbitA and RabbitB who are bonded, bond with RabbitC (trio)
> RabbitA and RabbitB might lose their bond, and refuse to bond ever again in the future (3 single cannot bond rabbits)
> RabbitA and RabbitB lose their bond, but RabbitA bonds well with RabbitC instead (split bond, 1 bonded pair, 1 single cannot bond rabbit)
> RabbitA and RabbitB reject RabbitC (1 bonded pair, 1 single)

First step is getting everyone neutered/spayed. Everyone must be hormone free. Do not start bonding until 2 months after everyone is fixed. Bonding early will likely result in serious injuries or even death from hormonal fighting

While everyone is fixed, healing, and draining their hormones, you can do prebonding. Youll want to put everyone in their cages about six inches apart from one another. This way they can see and smell one another, but cannot fight. Be prepared for possible spraying, territory pees, and territories poos around the cage. Leave them for a day or two so everyone is comfortable with territory. Once everyone is healed, start cage swapping. Bonded pair go into new bunnies cage and new bunny goes into bonded pairs cage. Leave them like this for about 2 days, then swap back. Keep swapping every 2 to 3 days. Again, be ready for territorial marking for a bit. Do this atleast until the 2 months are up.

Next step is bonding. Do not bond them until there is no aggression through the bars, and most of the territorial marking as died down. If you try to bond them too early, it will likely result in a fight. Also good to note that rabbits can and will hold drudges which can make rebonding very hard if not impossible in most cases. All bonding should start in a neutral space where no bunny has gone before. Start small, and progress to bigger areas, until the final space that they will be sharing. Do not progress to bigger spaces until there are no signs of aggression or tons of territorial marking.

Edit to add, I have a semi trio of lovely boys

I did a trio attempt at one time. I didn’t have success. A&B were bonded. I added C. After each bunny date, A would chase and nip B. I stopped trying. There are definitely members here who have had successful trios, I haven’t.