The three columns are not three equal boxes. They are three different jobs.
The left column is navigation. It tells readers where they are, where they can go, and what kind of publication they are reading. The center column is the document. It is the place where the writing gets the most room and the fewest interruptions. The right column is context: related reading, current sections, and the small signals that make an archive navigable.
When each column has a responsibility, the page can be dense without becoming confusing. Readers learn the grammar quickly, then use it to move through the site.