1. Screenshot Leak
This is the most recent leak to happen, seeing as it just appeared today on the Valve fan site, ValveTime. Some argue that it might not even be an accidental leak at all, but a purposeful action because of how careful the title of the folder is lined up with the top of the screen. The screenshot comes from a tutorial that Valve artist Tristan Reidford was doing when one of the folders on his drive was labeled "left4dead3." You can read our full story on the leak here.
2. Spotting Left 4 Dead 3 During An Office Tour
Back in 2013, IGN wrote up an article about an office tour of Valve's Bellevue, Washington location where people spotted evidence of Left 4 Dead 3. A Dota 2 player who was there for the International 3 tournament managed to sneak a picture of a changelog that included LFD3. In the image, the text says, "Restored LFD3's devtest unit test. Ran locally without an assert. There may be so..." I mean seriously, what else could LFD3 stand for other than Left 4 Dead 3?
3. Co-Creator Of Counter-Strike Admits It
This is probably the biggest piece of legit evidence out there so far. The co-creator of Counter-Strike, who no longer works for Counter-Strike anymore, has actually come forward to say that yes, Left 4 Dead 3 is in development, and even tossed in the fact that "it looks great."
So while we still haven't heard anything official from Valve yet, it's fair to say that there's an overwhelming amount of evidence proving that something is stirring at Valve within the Left 4 Dead franchise. So hopefully they won't wait too long to reveal it.