I just added a third pig on Monday. I had a pair of girls, about 2 and 1,5 years old, and I added another girl around 5 to 6 months old.
I washed them one by one and put them in a neutral area. Just a space none of them had used before with a big pile of hay. There was some chasing, posturing, rumbling and teeth chattering, but it went suprisingly well. (When they were still seperated I would get chattered and hissed at if my hands smelled like the new girl so I was a bit scared to put them together.)
After a couple of hours I put them in a totally new cage, so none of them had seen this cage. They explored some, did some more rumbling and teeth chattering and went on with life…
So far they are doing okay. There have been minor incidents, but the duo are still friends and the new girl seems to be mostly tolerated.
I think me washing them (with piggie shampoo) so they smelled alike and then putting them in a never been used cage helped a lot with them accepting eachother.