Seriously, they've never done it. Half Life 2 Episode 3 anyone? Also, Left 4 Dead 2 came out right after 1, but it doesn't look like they plan on making a 3. And no signs of a TF3, not that it's needed.
Main point: Discuss games that are taking/have taken way too long to make.
They will make Half Life 3. Episode 3 will never exist. There will only be Half Life 3. And when it comes out, it will be the game you'll tell your grandkids about.
Why? Because almost every game, except maybe Portal, that Valve made after Half-Life 2 was a test to their Source engine. They are editing their current engine, not starting from scratch. Every test that they do that is significant is released in a game. So if L4D and Portal are as awesome as they are, then Half Life 3 will be the best thing... ever.
It's weird, because Valve makes epic games, but by the second one, they've perfected it, unlike Assassin's Creed, CoD, Halo, etc.
I think that's a cool thing. The other games play chance with success every time they come out with a new game.
I think in most cases developers make new games when they have new mechanics and a longer story to tell. DLC doesn't necessarily need new mechanics or a super long story. We should be glad valve isn't pushing out games like activision, but I agree it would be nice for more of valve's games to hit the trilogy mark.