When natural light is incident on the first polaroid, the fraction transmitted will be 1/2 τ (only the component polarized parallel to the principal direction of the polaroid will go).
The emergent light will be plane polarized and on passing through the second polaroid will be polarized in a different direction (corresponding to the principal direction of the 2nd polaroid) and the intensity will have decreased further by
In the third polaroid the direction of polarization will again have to change by φ thus only a fraction will go through.