I cannot install my hommIII by galaxy, how to sort it out? is there any other way to install my game?

via browser goto https://www.gog.com/account, select the game and then "download offline backup installers"

In Galaxy, click on Library, hover over HoMM3 and a down-facing arrow will appear. Click on it and then click install. After it's done, it will show up on the left-hand pane. You can either right click and Play Now, or click on it, and then click the big button that says Play.

Both answers above are correct, I just don't understand what the eula has to do with not being able to install the game.

Eula be looking everywhere if you dont know where the installers are!

I cringed. +1 to eula, mate

and so you should! :D