HL3 has been on and off in development. You have to understand that Valve's development is mostly without a 100% focus on exactly one goal. So the development of what they do mostly change. Like F-Stop becoming Portal and HL2e3 becoming HLA among other cases.
Valve is predictable in the way that their games are only getting a sequel or an overhaul when it actually makes sense and isn't a quick cashgrab. VR didn't become as big as they thought so releasing HL3 in VR would've been a Blizzard moment ("Don't you guys have phones?") Tone deaf to reality. I'm not sure what the major innovation is going to be for HL3, maybe the photo realistic worlds, S2, enamgraph 2.5 it would be a combination of all of the good things from the Valve titles. If you want hints, then you should check out the bleeding edge, like Deadlock.
Valve is almost always at Siggraph, if not a speaker then an observer.
Now taking all the technically cool things, it does look like something is brewing, but I wouldn't hold my breath yet, Valve releases finished products, CS2 was forced out because the community requested so. (Someone had made CSGO in S2 a few months earlier so that was kind of shaking a lot of devs at Valve to hurry up).
As TF-S2 got pulled from S&box, it could be that we also get Source 2 in TF2 quite soon. I mean, TF2 doesn't even use OpenGL anymore, so maybe even next year, but lessons were learned from CS2 so it also could be 2026.
A lot is going to happen 2025, among other things, SteamOS, CS2 Operation, probably Deckard for christmas pre-order like Steam Deck and Index.
HL3 is a bit out, would expect it to be a bundle like with HLA, so when the Steam Machine or SteamOS releases around that time, so 25-26.
*I did predict many of these so maybe my professional opinion still holds water*