It is not that difficult to bond a trio I can't agree that rabbits do best in pairs. Maybe keeping two rabbits is easier for a typical rabbit owner that's why, but when you have more rabbits bonded together there's no pressure that they must act as a couple and groom only each other etc. There's much more freedom between them and it takes off the pressure. They are more relaxed in a group.
Anyway, if you want to bond them together, it will initially break your bonded pair, so I would suggest separating them all three and limit their space dramatically, say 100x50cm or 80x60cm cage for each of them and place so they can't see or smell each other. Let them enjoy themselves for at least one week, you will see how relaxed they will become. Small space, best covered with a blanket or something only leave front opening so they can feel like in a cave, they will feel safe, don't take them out, let them be. After they are ready you can try bonding all three same time in somewhere in a small space and it needs to be neutral territory for all three. Be prepared for little fur flying and humping just watch any signs of aggression and also don't let humping on head because you don't want that kind of injuries. Shake the space to distract attention if conflict escalates, you can use water spray bottle as well, let them have a little dinner together they will stop fighting, etc. Then if they are generally okay after 12-24 hours, depending on their personalities and you know them better so you decide when you can move to the next level. You can prepare their next bonding space, also not too big. Use white vinegar to remove smells, give them new toilet or make bedding with no toilet, placing wood pellets under it, mostly in corners, they will stay in that space for a couple days and after that you will see where their fav spots so will choose which corner use for their toilet box later.
Every bonding will be different so I can't tell you what to do step by step, just do some research on body language and watch some video I'll find one now for bonding 5 rabbits together, lots of useful tricks there.
It is totally achievable I have trios and even trio with two neutered males and one intact female, I am very proud for them they are so tidy and nice to each other but it takes time to get there, we had some fur flying and puddles all over the place and running wall to wall chasing etc, but all settled and now 3 months together, best of friends.
So you can do it too, please keep us updated and post some pics of them and their setup so we can criticise better