I like to take a peek at Blogging Basics 101 occasionally, and recently they had answered the question "Is it possible to add a third column to my Blogger blog?" The answer contained tons of links to instruct a person in making a three-column blog, and I thought it was very helpful.
Also, Mrs. B has done some beautiful work on her blog recently, and she wrote an entry detailing how she did it all. Go see what she's done and how you can do it yourself! When I have the time (what's that?!) I plan to look through it all myself and see if I can play on my blog a little more.