When the ‘trusted users can edit others’ setting is enabled, Trust Level 3 users should be able to edit topic titles and categories for categories that they have permission to create topics in. Trust Level 3 users cannot edit archived topics or personal messages. They can also not edit their own topics if the topic’s edit time limit has expired. Does this sound like what you are finding?
The code for this is here: discourse/lib/guardian/topic_guardian.rb at main · discourse/discourse · GitHub