To make a 3-Column layout, I used these two articles from Three Column Blogger:
You need to do both of them:
1)
Three Coulmn Minima Layout Version
2)
Adjusting Three Column Margins
*(1-2012) This information is pre-Template Designer .
To adjust the width of the layout using the Template Designer:
After you have
installed your Background and uploaded your Header go to:
Template > Customize > Adjust Widths > Entire Blog Width
Play with the width until it looks right. (It will be about 660px for a 2-Column and about 940px for 3-Column)