The standard 3-column layout is determined by the number of sidebars and block regions in the theme (not by Drupal). Some themes have as many as 18 block region

The standard 3-column layout is determined by the number of sidebars and block regions in the theme (not by Drupal). Some themes have as many as 18 block regions (header, banner, footer, content top, etc.) and blocks and widgets don't strictly have to be in the sidebar. It seems that you didn't know this, so I don't trust that your experience with Drupal is very authoritative.