I think you can do the introductions one after the other if you have time. My suggestion would be to "cross pet" first, that is, pet the new bun, then one of yours, then the new bun, then the other of yours. This way they will all smell a bit like the other bunnies, so your pair will not get mad at each other for smelling like a stranger. Do a short introduction with each of your pair and the new bun, watch closely for biting or chasing. If you have time and neutral space, you can do this several times a day.
Since this is a bunny that you are sure you want to keep, I’d suggest keeping the initial visits short (5 to 10 minutes, maybe you on the floor with them). You really do not want any early aggression to turn into a deal breaker.
Does the new bunny like to cuddle? One of the things that I did with my trio was to lie on the couch with all three bunnies on my chest. I got them nice and relaxed with pets and nose rubs, and sort of forced them into side by side positions with the new bun in the middle. This was a way to get them used to each other in a place where they could not get territorial because I was in control. Good Luck!