The community is also a great place to check out. Try these places for help from other users:
Please review the Third-Party Game Support topic to determine whether your question should be directed to the game developer or to Steam Support. Sending a support request to the wrong party may result in a delay in processing your request.
Retail CD-Keys for Dungeon Siege and Dungeon Siege 2 cannot be registered through Steam. If you wish to create a shortcut to this game under your Library, instructions can be found here:
Dungeon Siege 3 can be registered through Steam in this format: ABCDE - 12345 - FGHIJ
If you're encountering issues when launching or playing Dungeon Siege 3, make sure everything installed correctly.
Exit Steam entirely.
Browse to your Dungeon Siege 3 installation folder (Usually C:\Program Files\Steam\steamapps\common\dungeon siege iii\)
Run the following installers:
Reboot your computer and test the issue again. If the issue persists, contact Dungeon Siege 3 support.