Left 4 Dead 3 has been leaked following a picture that's been released that was taken during a tour of the Valve offices.
The picture, which was taken by members of the DOTA 2 subReddit as they enjoyed a tour of Valve's offices shows off a change log from a computer. It highlights Left 4 Dead 3, but also the Source 2 engine.
We already knew that Source 2 was being worked on, as Gabe Newell mentioned this towards the end of last year, but nobody outside of Valve knew they were working on Left 4 Dead 3.
No platforms were mentioned, but it's pretty clear that it would arrive on the PC as a bare minimum. However, with it likely to be using the Source 2 engine, as they appeared in the log next to each other, it could also be planned to release on the PS4 and Xbox One.