Keep digging, Watson!
File this one under "P" for "possibly real" or "possibly fake," but according to NoShitShurlock.com , development of Half Life 3 has already begun. The site claims it received confirmation from John Guthrie, a level designer at Valve and founder of Quake Command, after exchanging numerous emails with him. Half Life 3 is supposedly in the early stage of development, and the goal is to have a trailer ready for next year's E3.
"I can’t confirm all too much since the development is still in early stages, but for now I can confirm that we are going to release on both current and next generation consoles and we will also release the game on our own platform exclusively for PC and Mac," Guthrie supposedly stated in an email. "We hope to have the first trailer ready for E3 2014, which we (after several times not attending) will finally come back to."
Assuming NoShitShurlock isn't feeding the Internet a bunch of bull (and c'mon, what are the chances it's not?), Half Life 3 will include many of the main characters from Half Life 2 and continue where the story left off. Apparently Valve felt influenced by an improved game engine, next generation consoles, and new game mechanics to keep the franchise going.
Guthrie also said that Half Life 3 will be a single-player game only; no multiplayer or co-op.