It is just a matter of installing the flaps correctly.
Header Row: - 2 (3 * 2 colspan = 6 colspan-cells).
. - 3 (2 * 3 colspan = 6 colspan-cells).
UPDATE
, , , colspan, . , , ( ).
:
<table border="1" style="width: 50%;">
<tr>
<td style="width: 100px;">td1</td>
<td colspan="2">td2</td>
<td style="width: 100px;">td3</td>
</tr>
<tr>
<th colspan="2" style="min-width: 120px; max-width: 120px; width: 120px;">th1</th>
<td colspan="2">td2</td>
</tr>
<tr>
<th colspan="2" style="min-width: 120px; max-width: 120px; width: 120px;">th1</th>
<td colspan="2">td2</td>
</tr>
</table>
, th th th1 , td (td2). 50% , 240 , th1 120 .
, , , , - ( , ( imho)).