Break post into 3 linkable categories

I am moving my client’s site onto WP. For every topic he blogs about, he has 3 entries: past, present, and future. Seems like it would be better SEO to have one post (better content) and subdivided into the 3 categories.

1- i can #tag sections, is it possible to attach a tag to a category, so only that tag shows in the results list. ie I click on category “past” and i get a list of all the “past” entries, either titles or both titles and content?
2- if we do what he is doing now – 3 entries per blog (so 3 posts per subject) it be easy to look at a list with the 3 categories. Then how could we link the posts – so looking at any post, i would see the other 2 related entries? So if im reading “cooking methods-past”, i would see 2 links (like breadcrumbs) to “cooking methods-present” and “cooking methods-future”

We can do this manually, maybe im overthinking it… but there may be a theme or plugin with some sort of keyword or category linking that can be easily adapted for this.

Thanks for any input-