I'm not 100% sure but I think if you try to add a female to two bonded males they will fight over her even if both of the males are neutered, and even if she is spayed. Because neutering just removes the chance of making babies so the urge to fight over the female stays.
Like I say I'm not 100% but I'm fairly sure.
Three boys is unlikely to work, and adding a girl will just cause a fight. These things very rarely work when they are together from the start, but I've never heard of it working when you introduce a third. If you want more piggies, your best bet is the splitting the boys as @Squeakz suggests, but I would be loathe to do that if they have a good bond and are happy together. I would be more inclined to get another pair of boars.....just make sure you keep them apart.