According to Lowrie, Valve isn’t even working on Half-Life 3 at the moment. Responding to questions on his blog, Lowrie stated that one of the reasons Half-Life 3 isn’t being worked on is due to motion capture, which he claims does not work that well with video games yet. While he claims that motion capture translates better to movies where there is no interaction from the audience, “when you make something interactive it gets way less interactive with mo-cap. So that’s one of the things they’re working on.”
Of course we doubt Valve tells Lowrie all their plans and he acknowledges that he does not know what the company’s business plans are, but based on what he knows from a conversation he had, the game is not being worked on currently.