Using Different Themes In WordPress.com

If the blog that you are using is a WordPress.com blog, this is different from a WordPress.org blog in that .com are run by Automattic as a hosted service whereas .org is the same platform but run by users on their own sites.
With WordPress.com this means that you are limited to using only the theme templates that are pre-loaded. There are still plenty of choices, around 58 themes are available several of which are three column.
To access them you need to be logged in to your account at http://wordpress.com/
Your first screen should be similar to this:

As you look along your toolbar you will see “Presentation”:

Click on that tab and you will be taken to this page:

From here you can select any of the 50+ themes that are shown as you scroll down the page. Other things that you can do from here is add widgets to your sidebar and in some cases add your own header, as I have done on my Joss Whedon blog at:
http://whedonsworld.wordpress.com/

I hope these instructions are helpful, but if anyone is still having problems please feel free to leave a comment.