This is a first for me. I don't remember starting a post with "I don't know". The fact is, I don't. Take a look at the below thread, which was posted at the Norton Community forum. It may (or may not) clarify things.
According to the thread, they have a "3 User" and "1 User" option. If you're plans are to use it in 3 different households, it would lead me to believe, they may also view it as "3 different user's". I don't know what method they use to enforce their policy. It's ONLY a guess on my part.
"3 user vs. 1 user / 3 pc"
I'm sure you'll eventually get a response, from someone who's used Norton, or other software with the same sort of offer. If not, as you'll see in the thread, you can contact Symantec Customer Service for further clarification.
Best of luck..
C-Net says "Norton Internet Security 2010 (1 User, 3 PCs)". Does this mean that even if I buy the 3 PC pac, it can't be installed on 3 computers in different households?