<body onload="document.getElementById('idform').submit() ;">
<form id="idform" method="post" action="modifica.php">
<input type="hidden" name="primo" value="<?= $invio1?>">
<input type="hidden" name="secondo" value="<?= $invio2?>">
<input type="hidden" name="terzo" value="<?= $invio3?>">
<input type="hidden" name="invio" value="quellochetipare">
</form>

chiaramente in php
if ($_POST['invio'] == 'quellochetipare')
{ .....etc....

ciao