Would it even make sense to release a HL2:EP3 now, rather than a new game called HL3?
It's been so long since EP2 was released and I'm sure the new game will feature vastly updated graphics as well as some new gameplay mechanic (like that gravity gun did to HL2).
Half-Life 2 is Steam's daily deal for today.
Valve have already stated multiple times that they are done with the episodic model, so yes, the next Half Life release should be HL3 rather than episode 3. Which is likely why it's taking so long.