The offer from Apple is for users that bought a new iPhone (or iPad, iPod Touch, Apple TV box, Mac) from Apple or from an Apple Authorized Reseller, and haven’t enjoyed this offer before. The offer is available to accounts set to countries or regions where Apple TV+ is available. The offer is for the first registered user of the new device. Redeem the offer in the TV app on the new device, and find the “Enjoy 3 Months Free” redeem button there.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support
↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.