Is a 3rd column possible/advisable for the main index?

You could look at sidebar theme components for some existing options.

The search text is:
Sidebar #customization:theme tags:theme-component

The search URL is:
https://meta.discourse.org/search?context=topic&context_id=137907&q=Sidebar%20%23theme%20tags%3Atheme-component&skip_context=true