I think 18 is a bit late to get started but maybe they are doing things behind the scenes
I suspect all three girls are getting a behind the scenes education in a lot of thing relating to being a full-time working royal and what it means to be the sister of a monarch. Amalia isn't the only one who needs to know and understand how things work and what their position is going to entail. Doing Kings Day activities was a nice introduction for all three of them.
The younger two will also need career guidance and figure out what they want to do with the rest of their lives. Amalia needs to find something fulfilling to do while she waits until Wax steps down. As soon as she is done with school, she'll start university somewhere. And that will end her relative freedom.