The PC version of the Forza Horizon 3 demo will be available after the launch of the full retail game. That can be deduced from a blog post on the game's official website.
In the blog post Playground Games notes that the "Forza Horizon 3” PC demo will be released sometime after launch." The development studio encourages players to try out Forza Motorsport 6: Apex which is already available in the Windows 10 Store.
The development studio added that it is "dedicated to bringing the quality and innovation our fans expect from Forza to the PC" and that it is "confident players will have a fantastic experience with 'Forza Horizon 3' at launch.".
The full version of Forza Horizon 3 launches on September 27th 2016 for PC and Xbox One.
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