An anonymous source in Valve recently released some info on HL3. As of right now, the story, scripts, and overall outlook of the game is already done. However, the HL3 team only consists of less than 15 people and they don't even want to start on 3D model developement. Valve Management are making those 15 people move to other projects such as Source 2, CS:GO, and upcoming games. Why? Well, remember EoTL, Mass Effect 3, and Watch_Dogs? They were really ♥♥♥♥♥♥♥ ♥♥♥♥ yet incredibly overhyped. They got extreme backlash. EoTL started the whole "TF2 is dead" controversy, Mass Effect 3 gave the team at Bioware death threats, and Watch_Dogs was only popular on its release date. HL3 hype is almost 1000% above the hype of these. Valve is afraid that the community will have a huge backlash if the game was only mildly bad. HL3 will probably never get released. At least not in the next decade. On a sidenote, Valve is already earning more money per year than Scrooge Mc Duck in his entire life.
TL;DR HL3 will probably never be released due to Valve already earning an assload of money and because HL3 is just so hyped that the least bit of "bad" would make significant backlash that could hurt our beloved corporation.