<div class="left">左</div>
<div class="main">中</div>
<div class="right">右</div>

*{
        margin:0;
        padding:0;
        height:100%;
    }
    .left,.right{
        position:absolute;
        top:0;
        background-color:green;
        height:100%;
    }
    .left{
        left:0;
        width:100px;
    }
    .right{
        right:0;
        width:200px;
    }
    .main{
        margin:0 200px 0 100px;
        height:100%;
        background-color:blue;
    }

<div class="left">左</div>
<div class="right">右</div>
<div class="main">中</div> //注意中间部分的位置 如果把div放在中间 浮动上不去

 *{
        margin:0;
        padding:0;
        height:100%;
    }
    .left{
        background-color:green;
        float:left;
        width:100px;
        height:100%;
    }
    .right{
        background-color:green;
        float:right;
        width:200px;
        height:100%;
    }
    .main{
        margin:0 200px 0 100px;
        height:100%;
        background-color:blue;

    }