It's good that you've expanded their cage and that you're adopting instead of buying your guinea pigs

Last year, I had two boys that lived together in their 2x6 c&c. They got along very well. They slept together, ate together, and ran around together. They had a very good bond, and I was happy that they paired together so well. A few months later, I brought home a baby guinea pig. He was very timid and shy, and he lived alone for 3-4 weeks for his quarantine period. Then, I introduced him to the two older boys. Everything went great, there was some rumble strutting and a lot of veggie munching, and I put them together after thouroughly cleaning the cage that they were all to be living in together. I immediately started noticing differences in all the guinea pigs. The two older boys were a lot more skittish and frightened, and they would not allow the new pig to sleep in the same pigloo as them, and they ran to opposite sides of the cage as soon as he approached them. The younger pig did nothing but torment and annoy the two older boars. He was constantly on their backs, riding them around the cage. Eventually, things got pretty bad and my older boy Charlie would squeal every time the younger pig approached him. After a sharp nip was given to Charlie's nose by the younger boy, I separated him. Unfortunately, he's been living alone for quite some time since he can't seem to pair up with any other boar. I am discussing this with a local rescue, and I'll be bringing him in to see if there would be any male that would be suitable as a buddy for him.
I am not trying to discourage you from adopting a third pig, but you should just be prepared in case things don't go well.

Maybe you can ask your rescue if you can bring in your girls to see if they get along with some of the other females there? If one of your girls has already established dominance, then perhaps your situation will be different than mine. I wish you the best of luck, and hopefully you'll have a happy herd. :3