This is from John Patrick Lowrie's website. He did voice-work for HL2, L4D2 and DOTA2.
Sorry, Im afraid that the opposite is true. HL3 is not being worked on at this time as far as I know.
Right. As far as I know they are not developing HL3 now for several reasons, among them the mo-cap issue. Sorry for any confusion. What they might decide in the future depends on lots of different factors. I hope they do, personally, but it has to make sense for them.
Here is the biggest challenge with bringing out HL3: the big thing now with FPSs is motion capture, or mo-cap. One of the great things about HL2 is that all of the characters that you meet actually look at you when they talk to you no matter where you go or stand. With mo-cap you cant do that, at least not yet. Once you film the actor doing something and capture that motion, thats what the character is going to do. This works great in movies, but when you make something interactive it gets way less interactive with mo-cap. So thats one of the things theyre working on.
Still nothing definitive, but I hope this helps you understand at least one of the reasons why they havent brought out HL3 yet