Not going to happen, especially because Valve has expressed little interest in maintaining tf2 and is currently working on a class-based shooter (Deadlocked) that is not team fortress related.

Maybe in their initial planning for Deadlocked they considered making it tf3, but that ship has obviously sailed

it's been quiet...

Is this market about another game in the series, or "Team Fortress 3"? I think it's significantly more likely that they release a new Team Fortress game than "Team Fortress 3" (or any TF game with 3 in the branding)

@Sailfish another main line game in the Team Fortress series of multiplayer shooters