You didn't say if it was a fixed width or liquid width layout. The best way to see is by example.
Here is a good example of a fixed width 3 column layout:
Notice how the background is applied to the wrapper div and stretches across all 3 columns. This type of background scheme is only possible with a fixed width design.
Here is an example of a 3 column liquid layout:
Notice how there are 2 wrapper divs, each with it's own background and the background images are very wide (10000px) to account for the expanding layout