Behaviours are the bar are not indicative of whether they want to be together or not. It’s normal for separated piggies to interact at the bars, but doesn’t necessarily mean they can share territory.
It is worth noting that lunging is a defensive behaviour not an aggressive one.
Nipping (the biting with her mouth) is a gesture of power. It is not a reason to separate piggies unless it formed part is a long string of bullying. Bullying in sows can be more subtle and harder to spot.
If piggies didn’t like each other first time round and they were not able to form a bond, then they do not tend to change their mind about that. Once a bond has failed, reintroduction is not likely to be successful.
If you could provide us with a bit more information about your piggies, then we can help you further.
How big is the cage they all can potentially live in together? Three sows need a 5x2 c&c.
How old are they all?
How long have they been separated?
Other than lunging you’ve mentioned (as I say that is defensive, nor aggressive), were the any other behaviours which caused you concern and prompted the separation?
If you wish to try a reintroduction because they did not actually fall out previously, then you just put all the piggies in a neutral territory bonding pen. No hides should be in the pen, just a pile of hay and some water. You then add the pigs and monitor them for several hours. If successful, then you can clean down the cage they are to live in and move them back to it after several hours in the bonding pen. If unsuccessful and they don’t want to be together as a trio, then they will need to remain as pair and a single.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Sows: Behaviour and female health problems (including ovarian cysts)
Moody Guinea Pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour