In pet shops they often very young and/or litters and the shop has a limited time to sell them so they aren't often together long.
Ideally you shouldn't keep more than 2 boars together, trios are notoriously tricky and often the bond is broken when they fall out and you end up with 3 lone pigs. This is due to them hitting their hormonal phase at around 3 months which can last as long as 15 months and they fight to be top pig which will never be successful in 3's. In pairs, one will eventually submit to not being the top pig and follow the others lead. That being said occaisonally trios work but you don't hear of it very often, for the above reasons.
Pairs are successful as there aren;t multiple pigs fighting to be the leader. Also, in the wild, you wouldn't have more than one boar in a herd, there would be one boar with many females.
When you have a pair, and the right pairing, you will often have one more laid back pig than another, or maybe an older one who assumes a father/brother role to the younger one. Their always has ton be a heirachy you see.It's all down to personalities too.
In my personal experience, a trio is not worth the hassle, pairs are far much more successful!
