Download "Heroes of Might and Magic III" by using Steam. Steam is Valve's video game distribution platform. While the game costs money, Steam itself is free to install.
To purchase "Heroes of Might and Magic III" on Steam, type its name into the search box on the top right of the company website, and click the game when it appears. Once on the individual page of the game, click Add to Cart, open your cart, and then click Purchase for Myself. Type in your payment information, and complete the checkout process. When complete, Steam offers an Install button that you can click to download the game.
Steam shows the download process as a line at the bottom of the screen. Click the line to see how long the game takes to download.