Code:
<if condition="THIS_SCRIPT != 'showthread">
That one would be the condition to show if NOT in showthread. If you want it to only show in index.php, forumdisplay.php and showthread.php, you would do this:
Code:
<if condition="THIS_SCRIPT == 'index' OR THIS_SCRIPT == 'forumdisplay' OR THIS_SCRIPT == 'showthread'">
show your ad
</if>