Drupal kind of forces the 3-column layout on you. It's not really very easy to extend (from what I've experienced). I've just never seen anyone have a good Drupal experience.
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.
What do you really want to do? Wordpress can be surprisingly flexible. You might want to try that (w/ some plugins).