How old are your boars right now?
If they all got along there should not be a third wheel kind of deal.
Messing with pig social structures can be difficult. My guess is that you want another guinea pig for yourself, not for your pigs. In this case, I would suggest adopting a bonded pair of boars that can live seperately if they don't get along with your existing boars.
I once had a bonded pair of boars that I tried to introduce a third boar to. One of the existing boars paired up with the new boar, leaving the other existing boar alone.
It's messy business and if persued should be taken with caution and lots of guidence from the local rescue
