main

/* stylesheet */
.left1 {float:left; width:10em; height:600px; background:#def;}
.left2 {float:left; width:10em; height:600px; background:#ff9;}
.main  {width:auto; height:600px; background:#cf9;}

<!--source-->
<div class="left1">left1</div>
<div class="left2">left2</div>
<div class="main">main</div>