There is almost certain failure going to occur if you try (fights). Doing the proper introductions isn’t going to make a difference to liking each other or forming a hierarchy.
Of course the decision is yours but it is a high risk move there is no benefit in disrupting their bond if it is a good one, for the sake of having a third piggy which is highly unlikely to work out for the long term.
At best, upon failure your pair will remain in tact in their relationship and can stay together and then you’ll have a single piggy whom needs his own 6ftx2ft cage and his own new friend), or it could break the relationship of your current pair as well and then you’d have three single piggies, all needing separate 6ft cages and a new friend each.
Added on top, your current boars are right in the middle of their teens - while most boar pairs are fine, their relationship isn’t out of the woods yet so I would personally focus on them and ensuring they have a good bond for life. We sometimes see boar pairs get to the end of their teens and then suddenly and unexpectedly it goes wrong.
But in answer the question at the end, no a boar trio would not work in your current set up. To even attempt a trio - as I said, we do not recommend to do so - your cage needs to be at least doubled in size. Each boar would require a square metre each, so you cage needs to be bigger than 32 square feet. Even if you could make a cage that size, space is only part of the issue, character compatibility is the most important part and it’s difficult to get that right in a boar trio
To get more piggies, the safest thing to do so that nobody gets hurt and bonds aren’t broken, Is to set up a second cage and have a completely separate pair of bonded boars.
You could not get sows if they are to stay in the same room as simply smelling sows can cause your current boar pair to fight With each other.
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?