Hello Peyton Richart,
Welcome to Microsoft Q&A forum.
Forza Horizon 3 has been delisted in 2020 and no longer in the Microsoft Store list. However, users that owned it should still be able to install and play through the Xbox app Library. May I know have you checked the previous answer about how to find the game in the library? May I know if you are able to find it there? If you can find it but having issue installing, please let me know.
Additionally, can you please go to account.microsoft.com/billing/orders and check if you can find the order history there?
I'm waiting for your reply.