HTML-Code: 
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
       "http://www.w3.org/TR/html4/loose.dtd">
<html>
	<head>
		<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
		<link rel="stylesheet" type="text/css" href="css.css">
		<title>Test Seite</title>
	</head>
	<body>
		<div class="Box1">Box 1</div>
		<div class="Box3">Box 3</div>
		<div class="Box2">Box 2</div>
	</body>
</html>
HTML-Code: 
.Box1{
	border: 1px solid #f00;
	margin: 0; padding: 0;
	width: 25%;
	float: left;
}
.Box2{
	border: 1px solid #0f0;
	margin: 0 25% 0 25%; padding: 0;
}
.Box3{
	border: 1px solid #00f;
	margin: 0; padding: 0;
	width: 25%;
	float: right;
}
Geht sogar in allen Browsern.