The EULA for Windows 7 can be accessed through http://www.microsoft.com/about/legal/useterms/default.aspx
Under section 2. INSTALLATION AND USE RIGHTS it states:
b. Family Pack. If you are a "Qualified Family Pack User", you may install one copy of the software marked as "Family Pack" on three computers in your household for use by people who reside there. Those computers are the "licensed computers" and are subject to these license terms. If you do not know whether you are a Qualified Family Pack User, visit go.microsoft.com/fwlink/?Linkid=141399 or contact the Microsoft affiliate serving your country.
The web page the the link in the EULA statement leads you to a page that states: You are a Qualified Family Pack User if you obtained an upgrade license to Windows 7 Home Premium in a product labeled "Family Pack" on the product packaging and media.
The way the installation worked on my two computers leads me to think that the second and third computer might need to be at the same physical space as the first and using the same internet connection during the installation process. Just a guess and only a guess.
I think they know that people travel with their computers and laptops. I guess, again, but the way it looks on my systems, there is nothing to stop the laptop, say, from signing onto another network when away from the home network. I think for the INSTALLATION to occur with the family pack, you probably have to install at the same network location. Again. A guess.