By far the easiest way that I have found to do 3 columns (or any other number of columns split over the available space in weird ways) is YUI Grids. There is a YUI Grids Builder to give you the basic layout. The following will give you a 750px wide basic 3 column layout (split 1/3 1/3 1/3) with a 160px left sidebar. Changing it to to other widths, sidebar configs and column splits is really easy.
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5 YUI Base Page
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YUI: CSS Grid Builder
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Footer is here.
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