On the realisation that you’re unfamiliar with colspan, I presumed you’re also unfamiliar with rowspan, so I thought I’d throw that in for free.
One important point to note, when using rowspan: the following tr elements must contain fewer td elements, because of the cells using rowspan in the previous row (or previous rows).
table {
border: 1px solid #000;
border-collapse: collapse;
}
th,
td {
border: 1px solid #000;
}<table>
<thead>
<tr>
<th colspan="2">Column one and two</th>
<th>Column three</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan="2" colspan="2">A large cell</td>
<td>a smaller cell</td>
</tr>
<tr>
<!-- note that this row only has _one_ td, since the preceding row
takes up some of this row -->
<td>Another small cell</td>
</tr>
</tbody>
</table>