I can tell you about my experience of introducing new pigs, if it is any help.
When Freddie's brother, Ringo died we wanted a new guinea pig as soon as possible because you could really see that he was lonely. We were considering all our options and eventually decided to get a young boar.
Freddie is a very laid back, friendly guinea pig, and very very gentle (he was very loving towards Ringo during his illness). Freddie is also rather...large. We chose a good sized young guinea pig called Wes and tried to introduce him - and Wes started being agressive! We had to take Wes out and we waited until the next day and tried it again. They were in the empty hutch so had lots of space, and Freddie was curious about Wes and went up to him, and Wes started chattering. So Freddie left him alone and went to eat some food, however Wes kept going after him and eventually had poor Freddie corned so we had to whip him out! Sadly, it didn't work out with Wes.
We then decided to go for an older boar, and we found Nelson at a rescue centre. He was also laid back and only slightly bigger than Freddie. They were a good match and their introduction went well. There was never a dominant pig in the Freddie-Ringo pairing, but we think that Nelson is possibly the dominant one now as he quite often rumbles and tries to hump poor Freddie! He's not too bad anymore, but he still does it from time to time. (We actually think this may have been a sly trick to get the best spot in the run).
So introducing a new boar to a single one worked well. Adding a third to an existing pair can be a bit more difficult because, in a pair, if there's going to be a dominant pig, you will usually only have one because one will 'beat' the other one to the post, so to speak. The problem with three is that two might decide they're boss, causing issues.
I'd try to pick your new piggy based on personality rather than age, as you can see from the Wes example, being a baby doesn't always mean he's going to be nice! A rescue/rehoming centre is a good place to go because they can usually tell you a bit about their personalities.
I introduced my two new girls (young) to my three other girls (they don't live together, but share a run) and that went well. There's not much difference in their age or size. They were sniffing each others bottoms for a while, but other than that they're fine! Ace isn't so keen on them, but she's not agressive towards them, and Amy seems to be 'mothering' the new piggies a bit - she follows them everywhere!
Try to introduce them in a place that is sort of neutral. Introducing them in the piggies hutch may cause the new one to feel threatened, or the older ones to try and protect their territory.
