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Half-Life 2 is currently 100% off for its 20th anniversary, plus a major update
(store.steampowered.com)
Bonus: it also seems that the episodes have been rolled into the base game. Full details of the anniversary update.
They have mentioned before that they gave up on episodic development, which tacitly ditches Episode 3. The episodes ended up not being that much easier or faster to make and in a time when PC games in retail was still kind of relevant, it was a pain to make, distribute, and get shelf space for.