Valve co-founder and current bigwig Gabe Newell (GabeN) will be holding an official Ask Me Anything session on The GabeN Subreddit tomorrow at 3PM PST/ 6PM EST.
GabeN may address the recent interview with an anonymous Valve employee that declared "Half-Life 3 is dead and never existed".
I'm sure Gabe will be absolutely flooded with questions about Half-Life and whether or not Valve is still a games developer or not, but that doesn't mean we'll get official answers. All we know is that Gabe will indeed show up, it'll really be him, and he'll answer a few questions. With any luck, we'll drop some huge bombs on the PC gaming community...but don't be surprised if he avoids 99% of Half-Life 3 questions.
At most I'd expect Half-Life 2 Episode 3 over any other Half-Life news.