下面代码构造div分为3烂，左中右，主要通过float浮动来设置，side靠左浮动，side1靠右浮动，main自动适应调整自身宽度。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<style type="text/css">
body{margin:0px;} //整个网页左右无空白
#side { background: #99FF99; height: 300px; width: 120px; float: left; }
#side1 { background: #99FF99; height: 300px; width: 120px; float: right; }
#main { background: #99FFFF; height: 300px; margin:0 120px; }
</style>
</head>
<body>
<div id=side>ddddddd</div>
<div id=side1>ffffff</div>
<div id=main>vvvvvvvvvvv</div>
</body>
</html>
