It’s funny how you’re all using Portal and Portal 2 as an analogy to how (Half-Life 2: Episode 3/ Half-Life 3) will differ from the episodic models.
Portal was just a test-game VALVe threw into the Orange Box to see how the community would like it. That’s why Portal 1 was bland in comparison to Portal 2, VALVe didn’t want to spend time, effort, and money into something the community might reject.
When everyone exploded over Portal, VALVe went all-out on Portal 2. This time around, they redesigned characters, models, and soundtracks to the style and feel that they had originally intended.
Half-Life 2 was not a test, and everything that’s in the game is the way they want it to look, so the episodes did not change much at all in comparison to Half-Life 2, because there are no restrictions depending on whether or not the community will accept or reject it. And I’m willing to bet that yes, episode 3 will be run on the same updated engine as Portal 2, but no, Episode 3 will not be very stylistically different from its episodic counterparts.
AS for Half-Life 3…nobody knows yet.
So why are we arguing over this?