The French website Journaldugamer has reported that an unnamed source close to Valve would declare that
Half Life 3 is actually being worked on and will be an open-world shooter due out in 2013. According to what stated, the game was changed several times during development and distorted by RPG elements that allow a great interaction with the various PNG, much to justify the change in perspective of pure shooters to this installment in the series that will be a real third game, and not just a sequel.
Up to official communications, we invite you to take the information with the pliers, but we will keep you updated.