Then it is a matter of using a table or some other formating.

For instance from that same website and using tables:

<table border="1">
<tr>
<td><iframe
src ="/default.asp">
</iframe>
</td>
<td><iframe
src ="/default.asp">
</iframe>/td>
<td><iframe
src ="/default.asp">
</iframe>/td>
</tr>
</table>

That would be 3 cells across with the same iframe in each.