Код:
<!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=windows-1251" />
<title>Untitled Document</title>
<style type="text/css">
#main{
width:200px;
}
#left{
position:absolute;
width:100px;
background-color:#FF0000;}
#center{
position:absolute;
left:100px;
width:100px;
background-color:#00FF00;}
#right{
position:absolute;
width:100px;
left:200px;
background-color:#0000FF;}
</style>
</head>
<body>
<div id="main">
<div id="left">1</div>
<div id="center">2</div>
</div>
<div id="right">3</div>
</body>
</html>
Древовидный вид