Verify that you are eligible. If you have enjoyed an extended free trial before, then you can’t have another one. Redeem within the first 90 days on your new qualifying Apple device (not on a different device), in the TV app. Find the “Enjoy 3 Months Free” redeem button there.
If the device is not purchased as new from Apple or from an Apple Authorized Reseller, then the device may not qualify.
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.