Welcome to the community!
Firstly, you should note that the additional line discount is not eligible on all plans. In addition, you should note that the discount isn't necessarily available indefinitely. As of October 2018, the additional line discount is only available for 24 months (see here).
If you wish to discuss whether your third line is eligible for that discount, you would need to contact customer service. Alternatively, you might consider sending @FidoSolutions a PM. Once they verify some information, they'll also be able to access your account.
Hope this helps 😀