HTML colspan in CSS

Question

I'm trying to construct a layout similar to the following:

+---+---+---+
|   |   |   |
+---+---+---+
|           |
+-----------+

where the lower row's empty area is filled by the bottom.

I could easily achieve this with td colspan="3"> if this were a real table, but since I'm just making a layout that looks like a table, I can't use table> tags. Is CSS able to accomplish this?