父级盒子无高度，子盒子浮动，高度自适应

    #box{
        border: 1px solid red;
    }
    #left{
        width: 120px;
        height: 300px;
        float:left;
        background-color: grey;
    }
    #main{
        height: 500px;
        margin: 0 220px 0 140px;
        background-color:skyblue;
    }
    #right{       
        width: 200px;
        height: 600px;
        float: right;
        background-color: grey;
    }

<div id="box">
    <div id="left"></div>
    <div id="right"></div>
    <div id="main"></div>
</div>

<div id="box">
    <div id="left"></div>
    <div id="right"></div>
    <div id="main"></div>
    <div class="clear"></div>
</div>