You know, I have a socially retarded boar, too. Lee is the friendliest pig to people, but he´s some kind of psychopath with other pigs. He attacks other pigs at the second they get near him. I don´t mean a small dominance fight. I mean that as soon as he sees a pig, he turns into some kind of monster and jumps right on the other one, even if it´s a small or submissive pig. If he is free and can go near other pigs´ cage, he attacks the bars and wants to break in.
I tried everything. Even in a large room with just one other pig at a time, he still beats the crap out of the other for no reason. I tried giving him floortime with my rabbit (they are about the same size), but he kept biting Toby and trying to mount him. Let´s just say Toby was pretty pissed off.
I think there is just no hope for Lee. He has no manners and doesn´t seem to be willing to learn. Actually, he hates other pigs so much, I think he is better off in a cage of his own. Remember Lee is a rescue and he´s had a rough time. He was the pet store´s breeding pair with a female. The sow was pregnant and they were ok on the Saturday, but on the Monday, when the owner opened the store, he found Lee sitting in a cage with a dead sow and six dead babies. God only knows what went through his little head during the two days he was surrounded with dead bodies. Since he was now "useless", not used to being handled and already an adult, the owner was ready to let him go as snake food, so he came home with me. That was two years ago.
I put two younger pigs in with Sneakers when Shoebox died. Sneakers was a fairly easygoing pig and the three of them got on fine. When Boots got a little older he and Sneakers did fight for a short time until Boots established his dominance, then they were all reasonably happy together. Boots and Sneakers never spent much time together but Sox shared himself between Sneakers and Boots.
I would give it a try with all three together. Let us know how they go.