The offer is for the first registered user of the new eligible Apple device. Invoice email or invoice Apple ID is not important for this.
The offer should be redeemed on that new eligible Apple device, in the TV app. You should find an “Enjoy 3 Months Free” redeem button there.
The offer is available to accounts that are set to regions where Apple TV+ is offered. Availability of Apple Media Services - Apple Support
Not sure if it matters for the offer if the device is purchased in a different country. It would matter for service/warranty, though.
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.