It's never going to get made anyway because Valve is a dedicated F2P community/microtransactions based software developer and digital storefront now.
Well, honestly I think this is the case.
I fully believe Ep 3 was in dev, but other things came up.. waiting on tech.. people moved onto other projects that were new or more interesting... TF2 took off, Steam took off, and dota2 became a real project.. one that Gaben got really addicted too.
The revenue from TF2 and dota2 and Steam itself means that Valve doesn't "need" Half-Life to be a huge revenue stream to fund the other projects, the other projects fund everything already.
From a financial standpoint Valve doesn't need the HL franchise at all. Yet, it feels like the goodwill to gamers type of thing for them to do. It's obviously something people want, but then again there's risk of living up to what is now pretty lofty expectations of what gamers would expect.