There are 6 species of Rosella and up to 19 sub-species. Most are not dimorphic, where the male and female have different appearances. At 3 months of age, I think it would be difficult to tell. They start out with female coloration and if male, that starts to show when the bird goes through each adult molt. The only sure way would be to have DNA tests run. Most vets offer that service and there are some kits where you can send the sample in yourself.
As far as talking, the best method is to talk to your birds a lot. They are not known for talking, but some will learn a few words. It doesn’t really work to just repeat a random word over and over. Talk to them and use the same phrases for each thing – Good morning, Are you hungry, Did you miss me, Are you a pretty bird – just a few examples. Birds vocalize as a way to communicate, so they learn words in order to try to communicate with us. They have to be interested in what you are saying and have a desire to make contact with you. They are much better at whistling. While they can learn to do both, since whistling is easier, once they learn to whistle for your attention they might not be as likely to learn words. So generally work on words before you work on whistling. Rosellas can be lovely pet, but they tend to be shy, and often prefer your shoulder. Always move slowly with them and never grab one from behind. They are prey animals and this is how a predator would catch one. I’ll give you the links to our training page as well as bird behavior page.
Teaching Your Bird
Pet Bird and Parrot Behavior
Thank you for asking Lafeber,