三列布局

<div id="app">
    <div id="middle">
        <div id="middle-text">
            middle <br>middle <br>middle <br>middle <br>middle <br>middle <br>middle <br>middle <br> middle <br>
            middle <br>middle <br> middle <br>middle <br>middle <br>middle <br>middle <br> middle <br>
        </div>
    </div>
    <div id="left">left</div>
    <div id="right">right</div>
</div>
<style>
    #app{
        min-width:600px;
        overflow:hidden;
    }
    #left,#right{
        width: 200px;
        background: gray;
        text-align: center;
        float: left;
        /* padding-bottom把盒模型撑开，margin-bottom把盒模型padding撑开的隐藏 */
        padding-bottom: 10000px;
        margin-bottom: -10000px;
    }
    #left{
        margin-left:-100% ;
    }
    #right{
        margin-left:-200px ;
    }
    #middle{
        width: 100%;
        float: left;
        background: pink;
        padding-bottom: 10000px;
        margin-bottom: -10000px;
    }
    #middle-text{
        padding:0 200px;
    }
</style>