Install this extension via the Extension Manager.
Go to Web > TemplaVoilà and click on the sysfolder where you store your ds / to.
Select Page templates and click on the Edit icon of the datastructure you want to edit:
Now edit the datastructure.
Example:
<meta type="array">
<beLayout>
<![CDATA[
<table width="99%">
<tr>
<td valign=top width="33%">###field_mainbox_2###</td>
<td valign=top width="66%">###field_mainbox_1###</td>
</tr>
<tr>
<td colspan=2 width="99%">###field_mainbox_3###</td>
</tr>
</table>
]]>
</beLayout></meta>
Go to Web > Page (and select a page which uses this ds, of course) and enjoy.

