Well it's Valve after all. They take there time when releasing sequels and it's always worth the wait imo.
Think it was like 2 years ago I remember reading about Episode 3 being set in some snowy landscape, so I thought it was gonna be released soon after that... guess not lol. Now I'm thinking since it's been 5 years since Episode 2 was released, they are now prob just working on HL3 and forget about Episode 3.