两端固定宽度，中间自适应结构：

<div>

<div>

<div></div>

</div>

<div>2</div>

<div>3</div>

</div>

.wrap{width: 500px;border: 1px solid;overflow: hidden;}

.main{float: left;width: 100%;}

.content{height: 50px;margin: 0 150px;background-color: #f60;}

.submain{float: left;width: 150px;height: 50px;margin-left: -100%;background-color: #6f0;}

.sidebar{float: left;width: 150px;height: 50px;margin-left: -150px;background-color: #06f;}

结果如下所示：