Hiya! What you're describing is called referred aggression and it is very common where a bonded pair can see/smell another rabbit. Because they can't attack the other rabbit to get it away from their territory, they turn on their partner instead. It can very easily escalate into a fight and I have known bonded bunnies have very serious fights as a consequence of this.
Having them next to each other and the way they behave there is no indication as to how they will behave when bonded, though, as bonding should always be done on totally neutral territory where none of the buns have a territory to protect. Are all the buns neutered? If any of them are not yet neutered, I would advise getting that done as the first port of call, and then keep the pair separate from the single bun until you are ready to bond them. Having buns in pens next to each other is not bonding so what you see now is no sign of how they will bond. While in some cases it seems to help a bond if the buns have been next to each other and got used to each other beforehand (although it's not something I've ever done) this is normally just between two single buns and the behaviour of your buns suggests that this isn't the way to go - referred aggression tends to only get worse, not improve, with time, so I would keep them separate until you are ready to bond them completely.
Good luck! X