Элементарно:
<!DOCTYPE HTML>
<html>
<body>
<style>
.left{
float:left;
width:50%;
height:100%;
margin-left:-50px;
background-color: #fcc;
}
.right{
float:right;
margin-right:-50px;
width:50%;
height:100%;
background-color: #ccf;
}
.fixer{
margin:0 50px;
height:100%;
background-color: #cfc;
}
.main{
height:200px;
}
</style>
<div class="main">
<div class="fixer">
<div class="left">left</div>
<div class="right">right</div>
<div class="center">center</div>
</div>
</div>
</body>
</html>