Get the file and download Era2.4.exe and run it.
Install it in program files\gog.com\Heroes 3 subdirectory . In other words, get the install in the Heroes 3 subdirectory.
After everything is installed, you should be able to play in the Heroes 3 subdirectory
with the file h3era.exe .
I've started the game but haven't played it.
Another way is to go to Tools and then to ERA START MENU sub under the Heroes sub, and run start.exe where you can find other things like editors.