Well, when I first tried to add my girl Khaleesi to my bonded pair Nina and Olivia (they had been together for about 7 months) things didn't really work out... I bathed them all together and did all of the proper introductions, but the oldest girls simply wouldn't accept Khal, they wouldn't bully her or anything, but every time she tried to get close to them the pair would run away. In this meantime, Khal became really shy and sad. So, I sepparated her from the girls and brought home another piggie (lol), Abigail, and presented her to Khal. Once I had two pairs (bonded Abby and Khal, and bonded Olivia and Nina), presenting them was WAY easier. Khal became more confident in her approach, since she had Abby to back her up, and Nina and Olivia stopped seeing Khal as one "intruder" and started seeing her and Abby as a herd, and finally accepted the new girls ^^
Well, I guess it depends on your girl's personality afterall, things can always don't work out at first, and you will end up with a third lonely pig (like I did), so you'll have to accept the possibility of ending with 4 piggies instead of three. Lol. It happens.
(Oh, and don't give up on your long-haired pig search. They're awesome. XD)