Ok, just tried it and used
<table border="1" width="100%">
<tr>
<td width="33%">a</td>
<td width="33%">a</td>
<td width="34%">a</td>
</tr>
</table>
and it worked. just make sure you UNCHECK the disable html.
you can see it here....for a bit then ill be removing it.
http://www.4thefort.com/modules/xoopsfaq/index.php?cat_id=14
oh and make sure you have your cache off when trying it