main

/* stylesheet */
.left  {float:left; width:10em; height:600px; background:#def;}
.right {float:right; width:10em; height:600px; background:#ff9;}
.main  {width:auto; height:600px; background:#cf9;}

<!--source-->
<div class="left">left</div>
<div class="right">right</div>
<div class="main">main</div>