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>