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 can't do that, at least not yet. Once you film the actor doing something and capture that motion, that's 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 that's one of the things they're working on.

Still nothing definitive, but I hope this helps you understand at least one of the reasons why they haven't brought out HL3 yet