Might be a bit late, but for all the other people who may read this:
After you copied over the game files, go into your HITMAN 3 folder in steamapps/common and delete
- Launcher.exe
- Retail/gameinfo.txt
- Retail/HITMAN3.exe
- Retail/EOSSDK-Win64-Shipping.dll (this one is the most important, as it forces the game to open in epic)
Repair the game-files in steam game properties window afterwards and it will work!