Link to Leaked Info

http://pixelapocalypse.ctrlalttech.com/whoops-tour-of-valve-leads-to-left-4-dead-3-leak/

"The internal change log contained entries for both Left 4 Dead 3, and Source 2. Left 4 Dead fans rejoice! While Valve took some heat for how quickly they pumped out a sequel to the original Left 4 Dead, it’s been a good long while now and I for one am ready to jump back into that world of zombie killing goodness. Gabe Newell had confirmed in 2012 that Valve had been working on a new engine for some time, so no surprise there."

L4D3 will use Source Engine 2