Recent evidence has pointed to Left 4 Dead 3's existence, as if such a thing were ever truly in question. Of course there's going to be another one, eventually. A photograph of a change log that mentioned "L4D3" and "Source 2" taken during a Valve studio tour has been deemed legitimate by multiple sources who spoke with Rock, Paper, Shotgun.
One of the outlet's anonymous sources also suggested we collectively take another look at a leak from earlier this year which referenced both Left 4 Dead 3 and Source 2 as well as -- wait for it, because that's all we know what to do any more -- Half-Life 3. This doesn't really change anything until we get some actual announcements, but, good to know.
RPS Verifies: Those Left 4 Dead 3/Source 2 Images, More [Rock, Paper, Shotgun]