It’s good that the shelter have said they will help if there is a problem. As has been mentioned, it is rare for more than two boars to be able to live together long term. Make sure you do your research on boar behaviour so you can spot the warning signs. Although they are over the age for teenage hormones, a change in environment can make them re-establish the hierarchy which can result in fights if one doesn’t back down into a submissive positio, so even though they have been together up until now, there is absolutely no guarantee it’ll continue. To stand any chance, you will need to make sure you have a very large cage (the recommended for two male pigs is a 180cm x 60cm cage but given you’ve got an extra boar, it may be wise to add on at least an extra 30cm (so 210cm x 60cm)), have multiple food bowls and plenty of open ended hideys (no one entrance houses or boxes because one piggy can end up being cornered).