div{ height:200px; color:#F00;}
.left{ float:left; width:100px; background:#00f; _margin-right:-3px;}
.right{ float:right; width:100px; background:#0f0; _margin-left:-3px;}
.center{ background:#333; margin:0 100px; _margin:0 97px;}


<div class="left">我是left</div>
<div class="right">我是right</div>
<div class="center">我是center</div>
自适应屏幕宽度的居中布局