Sonic the Hedgehog 3 & Knuckles (or simply Sonic 3 & Knuckles or Sonic 3 Complete Edition) is a platform video game which is the result of locking-on Sonic & Knuckles with Sonic the Hedgehog 3 using the former catridge's unique "lock-on" technology. It is essentially a large combined game of its two lock-on components and contains all Zones from the said games and also contains special features unique to this game. This would have been the original Sonic the Hedgehog 3 the developers intended but time constraints by Sega resulted in the game being split into two.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.