Valve is a very secretive company so it is hard to really know what is actively being worked on and when. However, right now it would seem that Left 4 Dead 3 is still actively being developed as new evidence came to light this week, with the upcoming game showing up in a Valve employee desktop screenshot. On top of that, there has been a lot of updates made recently to the private beta of Left 4 Dead 2, leading to speculation that the private beta may be being used as a cover for beta testing Left 4 Dead 3.
Valve employee Tristan Reidford, posted a tutorial on importing assets into Valve's new ‘Destinations' platform and after some digging around, a screenshot of Reidford's pinned Windows 7 folders was spotted and right at the very top was Left 4 Dead 3. As Valve News Network points out, previous leaks have tied Reidford to Left 4 Dead 3's development in the past.
That wouldn't be a ton of information to go off on its own though. Reidford has been putting quite a few hours into Left 4 Dead 2 recently. According to SteamDB, the private beta for Left 4 Dead 2 has been getting a lot of updates in recent months, so it is currently being speculated that perhaps Valve are using the private beta as a front to test out Left 4 Dead 3.
The original screenshot of the Left 4 Dead 3 folder has since been replaced, indicating that this was actually an accident on Valve's part and that they did not intend to reveal or tease Left 4 Dead 3.
KitGuru Says: When it comes to Valve news, a lot of it comes down to speculation. We can't prove that the Left 4 Dead 2 private beta is actually being used to test Left 4 Dead 3 but the circumstances surrounding this screenshot and its eventual replacement seem to indicate that the game is being actively worked on. That said, Half-Life 3 has also been ‘worked on' in various stages for over a decade now so who knows when this game may actually see the light of day.