<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style type="text/css">
html,body{
height:100%;
}
*{margin: 0; padding: 0;}
.content{ 100%;height:100%;}
.wrapper{
position: relative;
z-index: 20;
100%;
height:100%;
}
.left-bd{float: left; 200px;height:100%;background: blue; }
.right-bd{float:right;300px;height:100%; background: green;;}
.main{position:relative;margin-left:200px;margin-right: 300px;z-index:30;top:-100%;height:100%;background: red;}
</style>
</head>
<body>
<div class="content">
<div class="wrapper">
<div class="left-bd">left content</div>
<div class="right-bd">right content</div>
</div>
<div class="main">main content</div>
</div>
</body>
</html>