PHP in Templates and Template Conditionals

[Destroy666]

1. - probably caused by 3. You have a redundant ) somewhere.

2. and 3.

<if in_array($mybb->user['usergroup'], array(1,2,3)) then>
<if $mybb->settings['enablepms'] == 0 then>
{$lang->pms_disabled}
</if>
</if> 

1,2,3 - comma separated group ids which aren't allowed to view PMs.
Not sure why you want to use $mybb->settings['enablepms'] for that, so the result of the code I posted may be unexpected for you.