In the diagram shown, a monochromatic light is placed in front of polaroid (I) as shown below.
As, per the given question, monochromatic light emerging from polaroid I is plane polarised. When polaroid II is placed in front of this polaroid I and rotated till no light passes through polaroid II, then I and II are set in crossed positions, i.e. pass axes of I and II are at 90°.
When a polaroid III is placed in between I and II, no light will emerge from II, if pass axis of III is parallel to that of I or II. In all the other cases, light will emerge from II.