How to align 3 divs (left/center/right) inside another div?

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.