You can redeem multiple 3-month (or whatever) pre-paid memberships and they will stack up. Personally, I go to the Microsoft Accounts site and enter these codes from the subscription management area. In the case that you’ve set up a credit card for reoccurring billing, these pre-paid membership codes will basically push your billing date back.

https://www.amazon.com/Xbox-Game-Pass-Ultimate-Membership/dp/B07TFP7JFH