With that CSS, put your divs like so (floats first):
<div id="container">
<div id="left"></div>
<div id="right"></div>
<div id="center"></div>
</div>
P.S. You could also float right, then left, then center. The important thing is that the floats come before the "main" center section.
P.P.S. You often want last inside #container
this snippet: <div style="clear:both;"></div>
which will extend #container
vertically to contain both side floats instead of taking its height only from #center
and possibly allowing the sides to protrude out the bottom.