It depends how you download a book -- like knc1 said, if you downloaded the book with your K3 and your K3 doesn't have the latest firmware, you cannot get AZW3.
You can also download directly from
http://amzn.com/myk for your PW3 if you don't want to get KFX on your PW3.
...
None of this helps if the book is old enough that it was published before AZW3 was developed.
Some books are just only available in MOBI.
In that case, you can import the book into calibre and convert it from MOBI to AZW3.