Well first off, the new one should have its own cage either way. For quarantine, and then also to not intrude on your bonded pair's relationship. I know I wouldn't want a room mate to move in with my husband and I. Male OR female, and we're "social" animals too.
Why do you want a 3rd? For you or for the other birds? Sounds like you want it for you, to me. Which is a good thing, I assure you.
Have you considered just keeping #3 in its own cage and having it be YOUR little buddy? Cockatiels are just the BEST.
But to answer you(sortov, since all are individuals): I have a 17 and a half year old grumpy old man cockatiel, and only just recently brought home a younger bonded pair. They are still in quarantine in the other room, but the male in the pair loves talking to my Alex bird. Mostly Alex is talking to me and the other bird trying to butt into the conversation, but Alex has had a few "chats" with the other male thru the house via contact calls. None seem upset about the others.
Sorry I can't give more advice. Other than a short period many years ago where I fostered another Cockatiel for a couple weeks, Alex has always been the only tiel in the house. I used to take him to work with me sometimes at the parrot shop(before I knew better), but the tiels there were all babies(lol, my boss scolded me even. "Why are you bringing a beautifully feathered adult cockatiel here? Makes all the babies look especially ratty", baby tiels are ever so good at messing up their feathers as they putter around).
From phone, sorry for poorness of reply