Since this is "abandonware" hosted on third-party archive sites and forums, many "Forza Horizon 3 Dev Build Download" links are infested with malware or adware.
Before searching for a download, it is crucial to understand the significant downsides and risks involved: forza horizon 3 dev build download
A developer build, also known as a debug build or internal build, is an early version of a game or software that is used by developers for testing and debugging purposes. These builds are often not publicly available and may contain experimental features, bug fixes, or work-in-progress content. Since this is "abandonware" hosted on third-party archive
Downloading and using this build is considered and comes with significant risks: Downloading and using this build is considered and
(Version 1.037.2) as a 53GB update for PC players. Unlike the intended 1.5GB patch for the Rockstar Energy Car Pack, this massive download granted public access to internal development tools, debug menus, and a significant amount of unannounced content. Historical Background and Incident