A recent Twitch interview with Minh "Gooseman" Le includes some quotes from the Counter-Strike co-creator and Tactical Intervention creator about Half-Life 3, saying he has seen concept art from the shooter sequel, and that Valve is "working on it" (thanks NowGamer). The ex-Valve employee starts by saying "I don't know if I can talk about that," before doing so, ending by saying he thinks that it's "old news" and that "everyone kind of knows it's being worked on." He then goes on to express his excitement over another unannounced project, the "new Left for Dead." He jokes about being in trouble for the Half-Life revelation, but also clearly thinks that both games have already been announced in some form. Here we have transcribed the parts pertinent to the two games:

"I think it's kind of public knowledge that people know that it is being worked on, and so if I say 'yeah, I've seen some images, like some concept art of it,' you know, that wouldn't be big news. I guess I could say that I did see something that looked like kind of in the Half-Life universe. It wouldn't surprise anyone if I said 'yeah, they're working on it,' so I guess to go on a limb, I would say yeah, I did see some concept art for Half-Life 3."
...
"The one thing I am really excited about is Left 4 Dead, the new Left 4 Dead... It wouldn't be any news if I said I saw it and it looks great. So, I was really excited when I saw that, I was like 'wow, this looks great'. Because I really loved Left 4 Dead, it's just one of those games that changed the industry."