It depends what angle the third one is at...

For polaroids to cancel all the light, they must be PERPENDICULAR to each other (well, the flat planes of the polaroid elements would be parallel, but the (imaginary) line on each lens that represents the vibration direction of the light that is "passed" needs to be perpendicular (say one up-down and one left-right).

This is the condition known as "crossed polars" and leads to extinction UNLESS an optically active material is between the two polars.

If the third element lines up exactly with one of the first two, you get the same effect as just two lenses, but if it is at a random angle, then:-

1) The light gets filtered by the first element so it is vibrating left-right.
2) It gets filtered again by the second element so that it is vibrating at the random angle.
3) It gets filtered yet again by the third element so that it is vibrating up-down.

Because none of those angle changes were exactly 90 degrees, some light gets through. It's dimmed, but not extinguished.

In mineralogy, geologists examine rocks that are sliced, stuck on a glass slide, and then ground extremely thin (30 microns). At this thickness, most minerals are transparent, and one can study the optical properties of the crystals which generally rotate light and give interesting effects between crossed polars. The details of the colours and the way they fluctuate as you rotate the slide are diagnostic for many minerals.