ND filter question

Didn't see an answer in the great FAQ on filters.
In a nut shell, are they stackable? My wife bought me one not knowing they are different. The 3 stop was not great on a sunrise over the ocean to get the blurred look I wanted. If I buy a 6 stop do I have the option of using both to get to +/- 9 stops?
Can I get three for the price of two I guess.
Thanks.

Yes, with a couple of caveats. Stacked filters may produce hard vignetting on wide angle lenses, and you will get increased image degradation (whether this is noticeable depends on the quality of the filters).