Someone might have already done this before and might be able to give you an example of what they did.
Assuming you want to learn rather than have someone do it for you...
When I want to learn something, I just create some files in /test directory and play with them there, leaving the real site up and running unchanged until I have it figured out.
To help understand one of the ways to do this with CSS. you can look at:
http://cumulus.tnetweather.com/test/3column.php
This is a bare minimum setup to show how you could set up three columns and to understand how they work. You can then tweak this using something like Web Developer (plugin for FF or Chrome) and change the CSS values live to see what they do when you change them.
Find what you like, you can the copy the CSS you have modified, and then save it in the css file that the test page uses so when you reload it has your changes.
There are two files that make up this test...
The css file which you can just click on the link and copy to your site
and the php file which you can click on the source link and copy that and save it to your site.
Note, even though this is a php file, it really has NO php in it, so you could just save it as an HTML file instead. I create all files as php in case I add php content later.
You can put these into ANY sub-directory and they will work as long as they are in the same directory each.
Files:
http://cumulus.tnetweather.com/test/3column.css
http://cumulus.tnetweather.com/test/3column.phps
There is a link in the page where I got the original content from when I first set this up for showing someone else. It goes on to do more concepts which you might find informative. He actually changes content-1 into a left side menu.