I would like to add a third column with check boxes to a table so that respondents can note if their answer is exact or estimate. How can I do that?

Hi,

The default Matrix field can only accept a single input type.  You could setup a second matrix next to the first one with checkboxes instead of text boxes. I setup a quick test form to show what that would look like:

http://support.jotform.com/form/51735736957974

This can be modified to make it look cleaner.

Or, you could use something like our configurable list widget:

http://www.jotform.com/help/282-How-to-Set-Up-the-Configurable-List-Widget

This will allow you to have multiple field types in the same widget.  Slightly different setup than that matrix however.