Auto-categorize latest N posts

I want my blog to show the latest post full-width, the previous 2 side-by-side below, and the remaining posts linearly below that. I know I can do this using categories and a theme like Divi. So how can I get this to happen each time I post:

Assign new post to cat. 1, post 2 to cat 2, post 3 to cat 3, and uncategorize post 4 (which was post 3, are you with me?). Ideally this should be parameterized to support any value N for the number of latest posts to be so identified. I see the desired visual effect on many blogs, surely they are not all having to remember to manually recategorize the last few posts with every new post? Apart from the danger of forgetting to do this and so hiding some posts altogether, how could you post this way by email? Thanks! Dick