If can try adding a
<br> before the name of the sub board.
Change:
$subcats_linktree .= '<a href="' . $scripturl . '?action=classifieds;cat=' . $row3['ID_CAT'] . '">' . $row3['title'] . '</a> ';
to:
$subcats_linktree .= '<br><a href="' . $scripturl . '?action=classifieds;cat=' . $row3['ID_CAT'] . '">' . $row3['title'] . '</a> ';
or you can put a separator (|) before or after the name of the sub board, for example:
$subcats_linktree .= '<a href="' . $scripturl . '?action=classifieds;cat=' . $row3['ID_CAT'] . '">' . $row3['title'] . '</a> | ';
or something else...