Forza Horizon 3 was officially from the Microsoft Store on September 27, 2020, due to expiring licenses for cars and music. Because it was never released on Steam, you cannot purchase it through any standard digital storefront today.
Forza Horizon 4 and Forza Horizon 3 Ultimate Editions Bundle (Xbox One, PC) - Xbox Live Key - GLOBAL where to buy forza horizon 3 pc
You may find remaining unused keys on sites like G2A.com or Driffle. Forza Horizon 3 was officially from the Microsoft
These keys are increasingly rare and often sold at a significant premium, with prices sometimes exceeding $200–$600. with prices sometimes exceeding $200–$600.