Conditional only if in childforum

[Kwikms]

Lets say I have 6 forums:

Main-1
- Sub -2
Another-Main-3
- Other-Sub-4
- Another-Sub-5
-- Sub-of-Another-Sub-6

I am looking for a way to check, if a forum is a childforum of a certain forum.

Like this, but I only want to put in the main forum.

<vb:if condition="in_array($forum['forumid'], array(3))"></vb:if>
<vb:if condition="$forum[forumid] == 3"></vb:if>

In this excample if I say ID 3 (which is "Another-Main-3") I want the conditional also be true if the user is in forum 4,5 and 6.

Any ideas how I can achive this?