On the GTPlanet forums, the head of the Slightly Mad studio Ian Bell confirmed the existence of Project CARS 3. He also told some details of the future game.
According to Bell, the third part will be inspired by the game Need for Speed Shift, which was also developed by Slightly Mad. The new game will be less a sandbox, so that the players were more fun. In this case, Ian Bell assures that Project CARS 3 will remain a simulator and will focus on realism. The physical model of the game should be even more detailed.
Project CARS 3 is still at a very early stage of development. The studio is still working on the concept, so details may change in the future.