Arma 3 is a truly massive military simulation sandbox game. It offers a wide variety of true/sci-fi combat situations in both singleplayer and multiplayer modes. It also comes out of the box with a very powerful editor that allows players to create their own scenarios, campaigns and mapmodes both for single- and multiplayer use.Go to: http://store.steampowered.com/ and download STEAM client
Click “Install Steam” (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don’t have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam…
Follow the onscreen instructions to complete the process.
After successful code verification go to the “MY GAMES” tab and start downloading.