Please try redeeming the offer on your new eligible Apple device — not on a different device. Find the “Enjoy 3 Months Free” redeem button in the TV app on that new device.
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.
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.