have been trying to set up 3 month free Apple TV after purchasing new MacBook Pro, but only getting option for 7 days

• Besides the eligible product, the user themselves also has to qualify for the offer. The offer is available if you haven’t enjoyed an Apple TV+ subscription before, including through a trial and through Family Sharing.
• For Macs, the extended trial redeem button only is presented after sign in to the TV app (menu Account).

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.