You can't chain conditions like that in PHP. Template conditions are basically transferred to PHP, so its basic rules apply.
You either need to do
PHP Code:
$GLOBALS[forumid] != 1 AND $GLOBALS[forumid] != 2 AND $GLOBALS[forumid] != 3
and so on. More elegant:
PHP Code:
<if condition="i$post['signature'] AND !in_array($GLOBALS['forumid'], array(1,2,3))">
BTW: OR condition will not work when adding up negative conditions. If you do "not in 1 OR not in 2 OR not in 3", then the condition will always be true. If it is 1, it is not 2 or 3, and if its 2, it's not 1 or 3.