fai così

codice:
<table width="450" border="1" cellspacing="0" cellpadding="0">
  <tr>
    <td width="33%"></td>
    <td width="33%"></td>
    <td width="33%"></td>
  </tr>
  <tr>
    <td></td>
    <td></td>
    <td></td>
  </tr>
  <tr>
    <td colspan="3"><table width="450" border="1" cellspacing="0" cellpadding="0">
      <tr>
        <td width="50%"></td>
        <td width="50%"></td>
      </tr>
    </table></td>
  </tr>
  <tr>
    <td></td>
    <td></td>
    <td></td>
  </tr>
  <tr>
    <td></td>
    <td></td>
    <td></td>
  </tr>
</table>