It’s not just about where they sleep, my three often sleep in the same house. They are a strongly bonded trio of sisters/half sisters/cousins (who knows?!) who have never lived apart. They need room to move around and explore as well as get away from each other some times, particular during their seasons.
Adding a third could also create an outsider situation with one feeling left out or even bullied, and not necessarily the new one. With a cage that size, I don’t think it’s worth the risk to your existing pair’s happiness