I keep seeing threads in the Model S area that are really somewhat generic about Tesla vehicles. For example, there's Tesla Moments (which I think should be in the Tesla Motors, but anyway...), and recently one about hill hold turning off after you leave the vehicle. I don't read the X or 3 areas, but I assume there are similar ones there, that apply to most Tesla vehicles. (Original and new Roadsters are probably exceptions.) Anyway, my suggestion would be a "Car postings common to S/X/3 vehicles" or something like that.
I agree this has long been a problem here, but I think the answer is fewer subforums, not more. I'm reminded of this xkcd. I tried to make the case for fewer forums here but I wasn't very persuasive. It's still crazy to me how much hierarchy we impose on content that is fundamentally not hierarchical -- e.g. to your point, Model S vs Model X vs Model 3 is an incredibly arbitrary distinction when discussing any number of topics, yet it triples the number of places those topics may appear. @Doug_G's advice to subscribe to all subforums you might possibly want to see was helpful, but in effect it just turns everything into one big forum which defeats the purpose of forums to begin with.
The forum has introduced a tags system, which should allow for cross-forum browsing. Don’t know if it solves the problems but it is an attempt to do so.