The reason for switching to a 3-column-layout is that in this way I can better exploit the space on the webpage. Having two columns where I can display information (on Blogger widgets) makes the page much more "reader-friendly". The more you can pack into one screen-size, the better it is. User's rarely want to scroll down endlessly, and therefore widgets placed further down on the page get less attention. Clearly you don't have to overfill it since then everything becomes messy :) .
When changing the layout I had to edit the XML-source of my blogger page. What is a little awful is Blogger's HTML code editor which is basically just a simple text-area. It would be nice if the Blogger team could implement a better editor which could support syntax-highlighting, code-formatting etc...The best solution till now is to just copy everything to a local editor on the computer, such as GEdit (on Ubuntu) or Notepad++ (on Windows), modifying everything there, and copying everything back online when done.
What is really perfect is Blogger's flexibility from which the GPC (Google Page Creator) team could cut a slice. You can change whatever you want and the graphical layout editor will recognize the changes. In this way you can add new widgets and content to your page without having to touch the HTML code again.
I would like to reference the http://blogger-hacked.blogspot.com/ site, from which I took the idea of a 3-column layout.