There are many ways to do this!
correct me if I'm wrong but the question is looking for this kind of result.
<table style="white-space:nowrap;width:100%;">
<tr>
<td class="block" style="width:50%">this should stretch</td>
<td class="block" style="width:50%">this should stretch</td>
<td class="block" style="width:auto">this should be the content width</td>
</tr>
</table>
The first 2 fields will "share" the remaining page (NOTE: if you add more text to either 50% fields it will take more space), and the last field will dominate the table constantly.
If you are happy to let text wrap you can move white-space:nowrap; to the style of the 3rd field
will be the only way to start a new line in that field.
alternatively, you can set a length on the last field ie. width:150px, and leave percentage's on the first 2 fields.
Hope this helps!