I don't think we'll be seeing Half Life 3, I think it's more likely we'll be seeing Half Life 2 Episode 3
There was Half Life, which had it's story and engine and gave birth to Counter-Strike, TFC, DOD, etc. All of those games were modifications of the Half Life game.
Then there was Half Life 2. It is built on the Source engine and the first installment was called Episode 1. It's simply the first part of a three part storyline. TF2, CS:S, DOD:S and more are built on the same Source engine but are no longer modifications of Half Life and are now their own independent games.
There's simply no way they're coming out with Half Life 3 before they come out with Episode 3.