Depends entirely on the specific character of the Rabbits. Introducing a third Rabbit may cause the original pair to fall out. But that is not inevitable. Knowing the character of the three individual Rabbits involved and noting the dynamics of the relationship between the pair first is always helpful.
I prefer to keep Rabbits in pairs or quads as I have always found that in trios one Rabbit is usually an outsider. But again that is just my experience, others may well have successful trios.