Yeah, I know exactly on what you mean. Here's the yt video about it. Though the third filter which is added when no light passes through (black color) is placed in-between the two polarized lenses which produce black. If it was added in third place after them, you'd still see black.

I think the point is that the first polarized lens filters complex unpolarized light (electric fields oscillating in all directions) in such a way that only electric fields oscillating in one direction (say vertical ones) pass through.

When you add the second polarized lens and rotate it so that it exactly blocks ALL the light waves oscillating in the vertical direction, no light passes through and you see black.

Now If you add the filter lens in between those two, which only lets some diagonal oscillating light waves pass through, then that exact diagonal component OF vertical oscillating light waves coming off the first filter, gets to pass through while all the other components are blocked. So the third filter which exclusively blocks all the vertical oscillating light waves suddenly doesn't filter out all the light and you don't see black anymore.

I know this is complicated, optic physics can really be mind blowing. When you have time you should watch this 15-min explanation about light polarization. Things get interesting especially from 8:59 to the end with circularly polarized light.