Assuming you have legally acquired the CS3_v0.9.zip or .exe file, here is how to install it on Windows 10 or 11.
The Internet Archive preserves old video game demos. You can find the original CS3_v0.9_Setup.exe from October 2018. Note: This version may lack multiplayer support but includes the full European map. Construction Simulator 3 PC Free Download -v0.9-
Construction Simulator 3
Trademarks are property of their respective owners. Assuming you have legally acquired the CS3_v0