<div id="left" style="float:left;">...</div>
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<div id="main" style="margin-left:[width of left col];">...</div>

<div id="left" style="float:left;">...</div>
<div id="main" style="float:left;">...</div>
<div id="right" style="float:left;">...</div>

<div id="main">...</div>
<div id="left">...</div>
<div id="right">...</div>