$i = 0;

while ($a = mysql_fetch_array($cat)) {
        if ($i % 3 == 0) echo "<tr>\r\n";
	echo "\t<td>" . $a[navn] . "</td>\r\n";
        $i++;
        if ($i % 3 == 0) echo "</tr>\r\n";
}
while($i%3){ echo "<td></td>\r\n"; if(!((++$i) & 3)) { echo "</tr>\r\n"; } };