I've successfully done a trio (adding a male to 2 bonded females) in the past. No problems with it upsetting the pair, we just all had to teach Norman not to be a buttmunch.
We've currently got four... We got Barnaby for Nala after Layne passed away... Alice got spayed at the same time so there were thoughts of trying a trio after hormones were gone. We picked Barnaby out online and when we got to the shelter, this rotten little blue eyed Dutch with the sweetest little nose wormed her way into our hearts. I said no fourth bunny, no way... but when we went back the next day (there was a delay in adopting Barnaby), I caved and Harley Quinn came home with us too.
HQ and Barnaby got on so well so fast that they were fast-track bonded in mere days. Unfortunately, Nala gets along at first but in longer sessions eventually tires of the other bunnies and gets nippy. Alice is the biggest hold up, as she gets way too excited and demanding about wanting others to groom her and when they ignore her, she gets bitey and fights can ensue. So, we have a pair and two solos right now. It's never caused any rifts in the bonded pair to do trio or quad sessions.
I strongly recommend looking for a shelter that allows bunny dating. Failing that, know your pair's personalities and what would be a good fit (I picked Norman out without help, so it can be done). Every shelter I've been to allows exchanges for a certain amount of time and if they're bunny savvy then they should understand why a new bunny might not be bondable with existing ones.