If you introduce a third boar, it will start a hierarchy battle between all three of them. If it doesn't work out (which is the likely outcome), it may be that your original two have fallen out so badly in the struggle for top pig, that they refuse to make up and be friends again - worst case scenario, that could leave you with three boars who don't get on with any of the others. If you have a stable pair, I wouldn't risk upsetting them. The only time I would recommend three boars is if they are already together when you get them, and they keep getting on - and give them tons of space. Even this has a 90% chance of failure in the teenage years, and you need to have a plan B. With boars, the best bet is - if they're happy, don't rock the boat. You could have your other boar in a cage next to them. This doesn't seem to bother them quite so much.
I have a trio (since babies) and a pair - all boars. When the new pair first were introduced to the room, it started a fight between the other three, which almost resulted in a separation (I managed to stop it before it got too serious and the fall out became permanent). My trio are now just over a year old, and I am hoping that they are nearly through the worst of their teenage hormones. They have a 5x3 grid C&C with a loft and plenty of space to get away from each other, but I have still had some pretty hairy times during the year, and there is very definitely an outsider of the three.
I remember another member asking about introducing a third boar, and deciding to take the chance - it resulted in a massive fight. Ultimately it is up to you to decide what to do, and we will all support you, however you do need to be aware of the risks and have a plan in case it doesn't work out.
