You don't seem to have specific column colours so you could just set the wrapper's background to white.
If you did have three column colours that needed to reach to the bottom then instead of using a white background on the above element make yourself a "faux column" image and repeat it down the y-axis of that wrapper above. The image would be 960px wide by about 5px high and would have the three column color backgrounds (and borders if necessary). As the wrapper always contains the three columns then it will always be the right height.
There are other methods of making equal columns but the above is still the easiest for a fixed width layout.