You need to understand that to nest a table inside another (put a table inside another table) its as easy as this:
<table id="table1">
<tr><td> <table id="table2">
<tr><td>this a cell from table2 which is nested on table1</td></tr>
</table></td></tr>
</table>
Now, to get a layout like you mantioned and starting from what I've already said,
Main table
<table> <!--one table-->
<tr> <!--one row-->
<td></td><td></td><td></td> <!--three columns-->
</tr>
</table>
Nested table
<table>
<tr><td>this</td><td colspan="3">is</td></tr>
<tr><td>an</td><td colspan="2">example</td><td>of</td></tr>
<tr><td>a</td><td>"big"</td><td>nested</td><td>table</td></tr>
</table>
Then, to nest the table just copy-paste the code from the Nested table on the cell of the Main Table wher it should go
<table> <!--one table-->
<tr> <!--one row-->
<td>col1</td><td><table>
<tr><td>this</td><td colspan="3">is</td></tr>
<tr><td>an</td><td colspan="2">example</td><td>of</td></tr>
<tr><td>a</td><td>"big"</td><td>nested</td><td>table</td></tr>
</table></td><td>col3</td> <!--three columns-->
</tr>
</table>
Cheers!