So if I understand correctly the client can create a new category with a maximum of three levels deep navigation? So I assume they are allowed to create the category “root” page with child pages of that root page up to three levels. And therefor you need the collection in the category root page so all child pages can be handled in a Twig template.
Am I still assuming right?
If so you can have Grav include default frontmatter when a page is created as pointed out in this post [ New pages with default frontmatter](New pages with default frontmatter).