This one's for you Eleanor
If you are not comfortable with changing your html to make your own 3rd column, then you will be happy to know that there is a faster and easier way to add a 3rd column (or 4th if you want it) to your blog. Blogger now has a DESIGN section on the dashboard that allows you to make some drastic changes that were not offered before.
And remember, you can preview as you go so there is no reason to worry about making changes and then not liking them once you see them live.
Here is the step by step
from your dashboard click DESIGN
then NEW TEMPLATE DESIGNER
Here you will find a world of options for your blog. You can pick and try as many as you want and none of them will change your blog until you hit APPLY TO BLOG.
(as a matter of fact I am playing with changes for my blog in the next window)
Now back to the 3rd column:
Go to LAYOUT
and here you will see many options for changing the look of your blog.
So go ahead. Try it out. Let me know how it went ;)