General Question about adding a new piggie to the pod

Hello! I'm the proud owner of two 7 month old sisters that I've had since they were one month old. They currently live in a 16 square foot one level habitat and also have free roaming floor time. I've been thinking about rescuing a young two month old female from a bad situation. I was wondering how different it is owning three guinea pigs vs. two. I'm completely aware of the need to introduce them slowly after a quarantine period and that there may be dominance issues to be sorted out. What I'm more interested in knowing is if owning three takes a lot more effort than owning two. If the effort is pretty much the same, I think I will go ahead and rescue the other piggie. Also, would my costs be drastically increased? I'm a student, but me and my bf do an outstanding job of giving them the attention and love and treaties they need.