For layouts such as this, I would strongly recommend using the Grid 960 system:
http://960.gs/[
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This will let you create three columns in a fixed width container - you can also create the header and footer with this layout system.
This is a very good approach - it is commonly used so you can get support if you need it and very easy to set up.
The page below has more detail on actually using 960gs - including how to deal with a header:
http://www.thewebmadeeasy.com/css/css3-grid-principals-3-creating-the-960-divisions-3/[
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Good luck!!