Thank you for your answer. What I'm looking for is three separate "pages" - one per category - that users can navigate to, where each "page" has a description of that category. ("Page" is in double quotes because it need not be a squarespace page, but even just a link to a collection of blog pages will work).
With your suggestion of summary blocks would I just create three separate front-facing pages, each having their own description at the top?
Further, don't summary blocks restrict the max number of items displayed to 30? Is there a way to increase that limit to show all posts in a given category?