Family Base is definitely a great feature to keep so it is important for all of your lines to be subscribed without any issues. What make/model phone is your third line? Do you see an option to add the third line under registration at www.vzw.com/familybase ?
Follow us on TWITTER @VZWSupport
If my response answered your question please click the _Correct Answer_ button under my response. This ensures others can benefit from our conversation. Thanks in advance for your help with this!!