In captivity, same sex parrots will often bond as mates and even behave as a male/female pair. It’s very difficult to keep adult parrots as a peaceful flock because it is simply against their nature. In the wild, juvenile parrots live as a flock and interact casually with each other until they are old enough to choose a mate. Once adult parrots have a mate, they will still live in a flock for protection, but any physical contact is limited to the mate, and flock mates that venture too close will be seen as a rival and driven away. Cosmo and Tango sound like they have formed a close bond, and Max isn’t going to be welcomed. This isn’t a matter of training, this is just nature. I’m not sure how large the cage is, but generally you need a large, outdoor walk in type aviary to try to house multiple parrots together. Even then, some parrots can be extra territorial and chase other parrots in the enclosure. The best thing to do is let Cosmo and Tango live together in their cage and Max in a separate cage. This is just parrot behavior and you really can’t go against natural instinct. While hormones probably do play a part in what is going on, it isn’t likely going to change when their hormones calm down. I understand they lived together in harmony for a long time, but this almost always happens with multiple parrots in the same cage. They simply are responding to their instinct to only have a close bond with one bird. We do get this question a lot, with owners wanting their pet birds to get along and interact and the short answer is this isn’t a realistic outcome. Any bird behaviorist will explain the same thing, and will cite their own birds as an example – some get along, but for the most part, each bird has his space and they can’t be left out together without supervision. I would not recommend that you continue to try to get them to accept Max again. He needs his own companion – either another bird or you.
Thank you for asking Lafeber,