Is it possible to create a category inside a subcategory, making a third level?
No, you can only have them 2 levels deep.
It has been discussed numerous times, it is pretty much a no-go. It is recommended to utilize tags when you need another level or organization. (and feature requests usually aren’t questions…)
The tagger plugin hasn’t been updated in 2 years, can anyone confirm if it still works.
Tagging is now integrated into the Discourse core in the latest beta
Is just hashtag then type the tag name?
Far more integrated and full-featured…
All features from here are basically now in the core.
Yes this has been discussed plenty, pick one of the linked categories.