Have you applied the voucher code that came inside the box packaging ?
If so, i would double check that when you applied the code to your account that the 3 month entertainment pass was accepted.
With your internet browser go to the NowTV website and click on My Account (top right of the screen) and on the drop down menu select My Passes.
Then check if your pass says that you are on offer with the 3 month duration, if there is a green "apply voucher" button still next to your entertainment pass, then try adding the voucher code again.
Welcome to the forum.
Couple of possible things. Firstly you may have activated the 14 day trial rather than your 3 month pass, or if you are checking your account in one of the apps rather than the website then some of the apps only show the date of the next monthly period rather than the actual end date of your offer.
On the NOW TV website go to your My Account > My Passes page
and check your pass end date on there. If it's showing your 3 month pass correctly you don't need to do anything else, everything will work fine. If it's just showing a 14 day free trial click the apply a voucher link and enter the code that came with your box again, this should add your 3 months onto the end of your trial.