colspan doubt

Hi:

I'm working with tables, and have a problem
How do I make this work??
This is the example:

<TABLE BORDER=0 width=100%>
<TR>
<TD COLSPAN="3">
blah blah blah
</TD>
</TR>

<TR>
<TD width=50% colspan=????>blah</td>
<TD width=50% colspan=????>blah</td>
</TR>

<TR>
<TD width=33%></TD>
<TD width=33%></TD>
<TD width=33%></TD>
</TR>

</TABLE>

Thanks a LOT.

- Gabriel.