Every time a slip, leak or whisper emerges from inside the caverns of PC shop owners and former developers Valve, the gaming news press (us occasionally included) gets into a frothy frenzy at the mention of Half-Life 3. "Could this be evidence they're working on it?!" everyone cries, tickling their own legs in excitement. OF COURSE THEY ARE! Of COURSE Valve is working on Half-Life 3! It's not even vaguely feasible that they wouldn't be! They've obviously been working on it, in some capacity, since HL2: Ep 2 came out. News would be if they actually bloody finished it.
Today's leak comes from the erstwhile spies at Valvetime, who have gotten hold of a Valve "project tracker", which contains words like "Half-Life 3", "Source 2" and "Left 4 Dead 3", and understandably the community enthusiast site celebrates this coup. But what the find isn't is revelationary.
What would be a hugely interesting news story would be discovering that Valve wasn't working on Half-Life 3, Left 4 Dead 3, or a significantly improved version of the Source engine. What would also make for front page headlines would be the news that they were actually going to bloody well release anything other than DOTA 2 for the 47th time.
So yes. You can assure yourself that Half-Life 3 is in development. It will have been in development in various iterations and forms for about six years (SIX YEARS since Episode Two), some of which will have been scrapped, evolved, persisted with and given up on. The only interesting thing left to hear about Half-Life 3 at this point would be news that they'd managed to make something worth talking about.