Many sources say that valve have changed HL3 development several times since it started back in 2007. But most recent information came from a close source to valve is shocking.

Apparently, Valve has decided to toss in some open world elements with Half-Life 3. Inspired by other games like Skyrim and Fallout, Half-Life 3 will feature NPCs that give the player quests to complete and a sandbox world to wander around in .
          Here we have a problem. Since original HL series was corridor style linear FPS some of the fans might not accept this at first.  All we have to do is trust VALVE on this and wait. Meantime hoping the rumors are true about 2013 release.
 Here are some leaked Artworks about HL3 and valve accepted that they are real stuff

Guess they are gonna make this "Single-player-worthy", seems worth the wait, all we have to do now is simply trust valve!!

i posted those on other topic long time ago -_-