With Viktor Antonov departure, I highly doubt there will be an Half-Life 3 anytime soon. He left saying he wants to work on a big project, involving interesting new creative ideas. And that Valve is pretty much only about Steam nowadays. Steam and money making. They may be releasing interesting titles once in a while like Left 4 Dead, but it was not enough for the Art Director it seems.
If Half-Life 3 were really in development, why would he leave? He clearly said Half-Life 2 was the last project he worked on that he really was excited about.