It wouldn't surprise me if Portal 3 is released at some point. I'm skeptical, but I'm not outruling it. The game is whacky enough that there are probably a lot of interesting and cool things that can be implemented into a worthy successor.
I am, on the other hand, utterly convinced that HL3 is not going to happen. The previous two were groundbreaking, stretching limits of what one can do with a physics engine. I'm having a hard time imagining that it can be pulled off a 3rd time, simply because I am unable to imagine any sory of content that would all:
a) fit with the series so that it still feels like a HL game
b) interesting enough to allow for the innovation that the previous two games had
c) good enough to justify a new game rather than just a tech demo
I sincerely hope that my opinion on the matter is simply a matter of failure of imagine, and that a good HL successor is released at some point, but sadly I think I'm right on this one.
Half-Life: Alyx is mostly what I hoped we'd get from HL3, inasmuch as it hits your points a & b for sure, and IMHO c (though I know that's not agreed on by everyone).
It had great action and expository setpieces (avoiding spoilers), and the (albeit relatively simple) puzzles definitely added something to Half-Life that really worked for me.
Unfortunately it didn't solve all VR issues (melee being an obvious one), and not least of which the cost. I played it on a cheap (~$100), janky old WMR headset, but not everyone can do that without vomiting, so a great PC and good headset are a hefty price, which is probably the biggest hurdle for a full-scale 3 in VR. Especially considering there just aren't many other games worth making that investment in, IMHO. I played the hell out of Alyx, a little of a few other games...but Alyx was the pinnacle of what VR could do for me.
Alyx did what most Valve games do, it advanced the industry. It is absolutely a half life game and it fits but it isn’t HL3. It isn’t that grandiose.
For people who accuse it of being a glorified tech demo, well, that’s exactly what Half Life 1 and 2 are. The sole reason for the existence of HL2 is just to sell the source engine to devs and to push Steam forward. It is a tech demo. Its puzzles are tech demos.
What Alyx did is implement proper gunplay and looting mechanics and really showcased how possible it is to tell a story in VR without taking your POV from you. I’d argue that there still isn’t a single VR game that nails one of the foundational pillars of Alyx as much as Valve did.
IIRC, Valve pretty much admitted that they really have no interest in making games anymore, unless they have a interesting technology to play with and learn, and the game is an excuse for that.
Thats why Alyx got made, cause they wanted to play with VR.