I also have 3 cockatiels and have been considering getting another for a little while. Two of mine are a bonded pair, so the other is often left on her own! As a result she has bonded to me, but I do wonder whether getting another tiel would be better for her and the flock overall.
Hmm that is a tricky problem. I would say if the third is bonded to you it should be left like that unless you can't be there that often for it. A fourth would work out fine if you weren't home as much as you think your 'tiel needs you.
Plus, if you get a super game 4th one than you will just have 2 bonded to you, but give each other company. And you third Cockatiel now that is the outcast will always love you even if you got another. I couldn't see anything wrong with that.. Anyways that's my thoughts. Give you 'teils my love!
Three is always a crowd. I started off with four for that specific reason. Even then, I had three males and one female (not on purpose mind you) and one of the males was STILL jealous because the hen didn't bond with him. I would say that if you can handle four, a fourth wouldn't hurt, but I would definitely recommend another hen to keep things even.