Aha! I think I have figured it out. With a bit more internet research, it turns out the third polarizer doesn't come after the first two, but must be inserted between them. I didn't get that from the initial video - which I think to most of us made it look like the last polarizer undid the cancelling actions of the first two.
So, to be clear, polarizers A and B are set to cancel each other out, then polarizer C is inserted between them at a forty-five degree angle and light is passed through.
However, this does seem a bit underwhelming - and not the "mind blowing" effect I thought I was seeing in the video.
Logically, polarizer A and C are at forty-five degrees, and so you get a half polarizing effect. Polarizer B at the back is another forty-five degrees rotated (at 90 to polarizer A) and you lose more light, but not all.
So, half of a half, is a quarter, and that looks like about what you get after the third polarizer.
I know, I get it. Theoretically, the rear polarizer should cancel out the front one entirely, but doesn't when there's another one in between. Still, it's a lot less magical than how it is presented in the video.
My gallery opening for landscapes taken with quantum entangled light has been cancelled.
---------- Post added 10-01-20 at 11:35 AM ----------
Originally posted by FozzFoster Now I'm thinking: why haven't I used two polarizers as a make-shift ND variable fitler!
Seems to me that'd work... haha
Actually, that's all a variable ND filter is. The trick is putting them into one thin frame so they don't vignette a wide angle lens.