HL3 is unlikely to ever happen. I can't see a groundbreaking innovation for the FPS genre in the near future that would meet the incredible expectations for, probably, the most hyped up game at this point in time.

We might get more Half-Life games once we reach the point of uploading your consciousness into the game, or similar. And even then those would be like HL: Alyx. Fun fact: Did you know modern VR headsets have been around for a decade, give or take? And they still aren't widely adopted.