At Generación Xbox we shared with you all the rumors about the potential development of Left 4 Dead 3. The latest news was very uncomfortable, because Valve himself confirmed that it wasn't working for a third installment of a joint shoot and that fact all hopes vanished.
To further clarify where this information came from the possible development of Left 4 Dead 3, Robin Walker and Chris Remo shed some light on this issue following the release of Hal-Life Alyx.
What was left 4 dead 3?
We used some parts of Left 4 Dead as the first level bricks to build with Source 2, but which we created online. Basically, it was a retrospective test, and people believed we were working on the new Left 4 Dead.
We built a few technological innovations behind Source 2… There were groups whose work was to do with what might have been a Source 2 goal, and some of them used items related to Left 4 Dead, but none of their creations came to the point that made us say "this is a great product we will build." It was a lot of Source 2 tools.
So, sadly, Left 4 Dead 3 didn't exist and Valve seems to have never had it in mind and knowing the company's history with third installations, it seems odd that we'll ever see a new installment of this beloved saga.