Ho risolto così:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css">
body{font: 76% Verdana, Arial, sans-serif}
a{color: #00f}
div#primo{display: none}
div#secondo{display: none}
div#terzo{display: none}
</style>
<script type="text/javascript">
function Show(id){
if(document.getElementById)
document.getElementById(id).style.display="block";
}
function Head(id){
if(document.getElementById)
document.getElementById(id).style.display="none";
}
</script>
</head>
<body>
<form name="form1" method="post" action="risultati.php">
<table width="300" border="1">
<tr>
<td>1°</td>
<td>2°</td>
<td>3°</td>
</tr>
<tr>
<td colspan="3">
<div id="primo">
<table height="300" width="100%" border="0">
<tr>
<td height="100">1a</td>
<td><input type="text" name="a1"></td>
</tr>
<tr>
<td height="100">1b</td>
<td><input type="text" name="b1"></td>
</tr>
<tr>
<td height="100" colspan="2"></td>
</tr>
</table>
</div>
<div id="secondo">
<table height="300" width="100%" border="0">
<tr>
<td height="100">2a</td>
<td><input type="text" name="a2"></td>
</tr>
<tr>
<td height="100">2b</td>
<td><input type="text" name="b2"></td>
</tr>
<tr>
<td height="100" colspan="2"></td>
</tr>
</table>
</div>
<div id="terzo">
<table height="300" width="100%" border="0">
<tr>
<td height="100">3a</td>
<td><input type="text" name="a3"></td>
</tr>
<tr>
<td height="100">3b</td>
<td><input type="text" name="b3"></td>
</tr>
<tr>
<td height="100" colspan="2"><input type="submit" name="Submit" value="Invia"></td>
</tr>
</table>
</div>
</td>
</tr>
</table>
</form>
</body>
</html>