My neutered boar, Billy, lives in with his grandmother and her friend, Muffin. Both of them stand up for themselves so when he gets a bit rowdy they tell him off. My Billy's a hard lad though and doesn't seem too fussed by their occasional nose-buts. All in all they live very happily together and have formed themselves into the sort of group you'd find in the wild. When out in the garden Billy acts like the dominant and leads them all around the yard!
From my experience trios work very well. I once had one neutered boar in with four sows and that went well too.