How to display only posts assigned to a particular, isolated, subcategory

I am trying to set up my wp site where I use posts assigned to categories and subcategories (about 3 levels). I have a list of 1st level categories as my primary menu. When clicked, the intended behavior is to take you to a level 2 subcategory landing page with an Intro post assigned to that subcat and a list of links to the 3rd level sub categories. Upon clicking one of these links, you are taken to the 3rd level category page which displays all of the posts assigned to that 3rd level sub category.

E.g.
any_page.php
_____________________________________________________________
Primary Menu – Categories: [Cars], Trucks, Trains, Boats
__________________________________________________________________

category.php
__________________________________________________________________
Level 2 – Cars Category Landing Page:
Cars Intro Post (assigned to cars cat) followed by
List of Title Links of Cars Subcats:[Ford], Toyota, Nissan, Mitsubishi
__________________________________________________________________

category.php
__________________________________________________________________
Level 3 – Ford SubCategory Posts
Posts assigned to the FORD subcategory: F250, Mustang, Falcon . . .
__________________________________________________________________

I have searched the web for weeks, to see how to achieve this, and cant find anything that really does this with posts (not pages). Some articles sort of touch on this but in those examples the links and posts typically are not isolated to the subcategory in which they are linked and you also get the links and posts assigned to the child and/or parent categories/subcategories too.

I am a bit of a noob at wordpress and PHP so any help would be great!