If Valve were to release Left 4 Dead 3 I would suspect its release date to be early November 2011. Video game publishers love to release new games just before the holiday season because that is when video game sales are at their highest. With high anticipations for L4D 3, Valve certainly does not need any holiday help to achieve recording breaking sales.
The Game Developers Conference is being held in San Francisco, CA beginning February 28th. If no news about Left 4 Dead 3 is released by the end of this conference then expect something to come out of the E3 conference in June 7th-9th.
Here is a list of games G4TV expects to be announced during this year’s E3 expo. Direct your eyes towards the bottom of their list you will find Left 4 Dead 3.