SOURCE

<div class="bd">
    <div class="left">left</div>
    <div class="main">main</div>
    <div class="right">right</div>
</div>

.bd{
    position:relative;
    width: 100%;
    height:500px;
}
.left{
    position: absolute;
    width:100px;
    height:100%;
    left:0;
    top:0;
    background-color: yellow;
}
.right{
    position: absolute;
    width:100px;
    height:100%;
    right:0;
    top:0;
    background-color: yellow;
}
.main{
    padding: 0 100px;
    height:100%;
    background-color: gray;
}