Hey Kevin -
The only way you'll get your columns to all be the same length is to set a static height on them (which you shouldn't do). Instead, what I would do is wrap an ID or class around your column container, create your background image that has the three column images within it, and set that to repeat-y. If your background image is a gradient from left to right, for example, then you can just create it 1px high and repeat the background image down the page.
This will give you the effect of having three columns with different background images, however, the length of the backgrounds will always be the same. Hopefully this makes sense...