$form_username = 'bleah';
$form_name = 'joejoe';

$SQL = "SELECT FROM table WHERE"; 
if ($form_username) { $SQL .= "username = $form_username AND "; }
if ($form_password) { $SQL .= "password = $form_password AND "; }
if ($form_name) { $SQL .= "name = $form_name AND "; }
$SQL =~ s/\sAND\s$//; #exists if not everything is filled in

print $SQL;