[livibetter]

Check this screenshot

If this what you need, then open your front-page.php and forum.php. Find

<?php while ( bb_forum() ) : ?>

<tr<?php bb_forum_class(); ?>>

<td><?php bb_forum_pad( '<div class="nest">' ); ?><a href="<?php forum_link(); ?>"><?php forum_name(); ?></a><small><?php forum_description(); ?></small><?php bb_forum_pad( '</div>' ); ?></td>

<td class="num"><?php forum_topics(); ?></td>

<td class="num"><?php forum_posts(); ?></td>

</tr>

<?php endwhile; ?>

Replace this (7 lines) with

<?php while ( bb_forum() ) :

global $bb_forums_loop;

$_loop =& $bb_forums_loop;

if($_loop->walker->depth == 3) : ?>

<tr<?php bb_forum_class(); ?>>

<td colspan="3"><?php bb_forum_pad( '<div class="nest">' ); ?>Subsubforums:

<?php while ($_loop->walker->depth > 2) : ?>

<a href="<?php forum_link(); ?>"><?php forum_name(); ?></a>

<?php

bb_forum();

if ($_loop !== null && $_loop->walker->depth > 2)

echo ', ';

endwhile;

?>

</td>

</tr>

<?php endif; ?>

<?php if ($_loop !== null) : ?>

<tr<?php bb_forum_class(); ?>>

<td><?php bb_forum_pad( '<div class="nest">' ); ?><a href="<?php forum_link(); ?>"><?php forum_name(); ?></a><small><?php forum_description();?></small><?php bb_forum_pad( '</div>' ); ?></td>

<td class="num"><?php forum_topics(); ?></td>

<td class="num"><?php forum_posts(); ?></td>

</tr>

<?php endif; endwhile; ?>

Note that this code prints out all descendants of a subforum together, not only in subsubforum level. If you need only subsubforums, then please use the code from http://www.livibetter.com/it/topic/example-3-level-forums-list?replies=1