They'll never release HL3. They are not a developer anymore. They are just a game store/directory. HL3 has been overhyped so much that anything released would be a disappointment. The gaming market has changed too much from when they made a game engine and released half life to showcase that game engine.
I can probably list a million more reasons why they'll never release, but those are the big points.
Half-life Alyx was HL3, just it was better to name it not HL3, because fans would lose their minds.
On the one hand, yeah. On the other hand, HL:A ended with an obvious sequel hook, and that hook was the ending of HL2:E2. Spoilers, I guess, but the game's been out for a while.
Of course, that doesn't mean another game is coming, but it does mean that HL:A doesn't mean another game isn't coming, either.