A me funziona:
codice:
<!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=iso-8859-1" />
<title>...</title>
<style type="text/css">
#overflow {
width: 250px;
overflow:auto
}
#contenitoreDivFlottanti {
width: 300px;
}
#box1 {
float:left;
width: 100px;
background-color:green
}
#box2 {
float:left;
width: 100px;
background-color:yellow
}
#box3 {
float:left;
width: 100px;
background-color:red
}
</style>
</head>
<body>
<div id="overflow">
<div id="contenitoreDivFlottanti">
<div id="box1">primo float</div>
<div id="box2">secondo float</div>
<div id="box3">primo float</div>
</div>
</div>
</body>
</html>