[tbworld]

Try this, I have not tested the code, so please let me know.

(Alter the post count ($tbworld_postbit_postcount >= 3) to what suits you.)

// Revised Last Post Advert
<vb:if condition="$post['islastshown'] AND $tbworld_postbit_postcount >= 3">
<li class="lastpost_advert_container"><div class="lastpost_advert">
      REPLACE THIS WITH YOUR OTHER ADVERTISEMENT CODE HERE
</div>
</li>
</vb:if>

Create Plugin to keep track of post counts per page.

PHP Code:

// Hook: postbit_display_complete
if (THIS_SCRIPT == 'showthread')
{
    global $tbworld_postbit_postcount;
    vB_Template::preRegister('postbit_legacy',array('tbworld_postbit_postcount' => ++$tbworld_postbit_postcount));        
} 


Should help.