Just create an action flag for the submit button and switch it in the IF POST.
if($_POST){
$id = $_POST['id'];
$item1 = $_POST['item1'];
$item2 = $_POST['item2'];
$item3 = $_POST['item3'];
mysql_query("UPDATE <TABLE> SET item1 = '" . $item1 . "', item2 = '" . $item2 . "', item3 = '" . $item3 . "' WHERE id = '" . $id . "'");
}
print "<table>";
$result = mysql_query("SELECT * FROM <TABLE>");
while($row = mysql_fetch_array($result)){
print "<form>
<input type='hidden' name='id' value='" . $row['id'] . "' /><tr>
<td>Item 1 : <input name='item1' type='text' value='" . $row['item1'] . "' /></td>
<td>Item 2 : <input name='item2' type='text' value='" . $row['item2'] . "' /></td>
<td>Item 3 : <input name='item3' type='text' value='" . $row['item3'] . "' /></td>
<td><input type='submit' value='edit' /></td>
</tr></form>";
}
print "</table>";