I think that the proof is in the (dating) pudding - whichever combination works best for your couple. Not all sows will accept every or sometimes any boar, but a third sow will also need to harmonise in order not to create an outsider problem in the long term.
Personally, I would try neutered boars first. They don't break up the existing hierarchy and need to get on with both girls. I had very good success with boar dating my two redoubtable 3 year old cataract sows with a submissive younger boar last summer.
You usually know within the first 15-20 minutes whether tings are working out or not. If I were you, I'd start with the rescue that had the biggest number of promising candidates for dating and keep an open mind.
Please be aware that trios are the most tricky constellation to get just right; if you have the chance at adding a younger bonded pair of sows instead of just one, do so - you will avoid the outsider problem that way.
