Not sure what question wasn’t answered.
As regards the wish for more than two category levels in general, this has been discussed in a number of topics on this forum and from what I’ve seen, there was always a way to do what people wanted to do without introducing additional levels of hierarchy, e.g. by using tags.
The simplest solution, however, is to reduce the number of categories when planning a new forum, regardless of how hierarchy levels. There is a great tendency among site admins (me included) to sort imagined future discussions into neat categories, perhaps even believing that the existence of these categories will encourage users to create content in these categories. But I believe the opposite is the case. A long list of categories scares people away.
Anyway, all this has been discussed before…