Please try to download and install Heroes3 HD mod:
During the installation you will be asked for the base game's installation folder. You can find it:
- If the game was installed via Galaxy, please select the game in Galaxy, click on More -> Manage Installation -> Show Folder
- If the game was installed via offline installer, then you will have to manually navigate to the game installation folder, by default it's: C:\GOG Games\HoMM 3 Complete\
Once installed, please run the game using HD_Launcher executable, located in the game's installation folder, or using a desktop shortcut to it.