There is a very good chance that an aggressive sow will not accept a boar either. At least, you can pair up the boar with the undersow and keep the single girl alongside.
Ideally, let them get used to each other next door first. You can try and introduce all three together on neutered ground etc. (Look up our introductions thread in the reference section for more tips.) Have a towel ready in case there are bloody fights.
If you haven't got a boar yet, arrange for some dating at a rescue to see whether there is a chance that it works out in the first place.