Hello Srinif1do,
I understand it's frustrating not being able to avail of an offer available to one of your other lines. However, certain promotional and loyalty offers are exclusive to the line and not necessarily to the entire account. I think there are many factors which are considered when identifying the offers available to a particular line. In addition to duration of service, the system would likely include factors such as usage statistics. Unfortunately, if the system did not offer that same additional data on the third line, I'm not sure whether it can be added to that line.
That said, you should note that this forum is community-driven and not intended as a venue for customer services. If you wanted to discuss your issue, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
