I don't believe there is an Episode 3.
Doug Lombardi mentioned that the initiative to produce episodic games failed, since they weren't really all that episodic, and they were considering returning to the monolithic development schedule. This was about three years ago. Combine that with Valve's obsession with quality, and you have the reason why Half Life 3 hasn't been released yet.
My advice, keep calm and be patient, or at least go to that rally being held outside Valve HQ.