It is generally easier to add youngsters to an existing piggy pair, as that doesn't upset the hierarchy so badly and minimises dominance. Please be aware that trios are the trickiest piggy constellation and the one that is most likely to end up with an outsider problem, as you need the character balance just right.
I would strongly recommend to either date at a good rescue for a neutered boar (provided that your girls are good friends, otherwise you'll end up with a 2+1 scenario) if you specifically want just a trio, or to go for two younger sows instead in order to avoid major headaches; the youngsters will have hteir own bond, so you neatly avoid the outsider problem.
Boars operate outside the sow hierarchy, so they don't upset the relationship between your girls as much, but you need to preferably let your girls choose their "Mr Right" or you can be in deep trouble! Considering how sociable piggies are, they are amazingly particular who they want to get on with! With sows, acceptance happens at the beginning, so dating at a rescue will show up within half an hour whether a relationship is going to work out or not.
