<xen:if is="{$thread.thread_id} == x">
This content will show in thread x
<xen:elseif is="{$thread.thread_id} == y" />
This content will show in thread y
<xen:elseif is="{$thread.thread_id} == z" />
This content will show in thread z
<xen:else />
This content will show everywhere else
</xen:if>