Three polarised sheets are co-axially placed. Pass axis of polaroids 2 and 3 make $30^{\circ}$ and $90^{\circ}$ with pass axis of polaroid sheet. If $\mathrm{I}_0$ is the intensity of unpolarised light entering sheet 1 , the intensity of the emergent light through sheet 3 is $\left(\cos 30^{\circ}=\sqrt{3} / 2, \cos 90^{\circ}=0, \cos 60^{\circ}=1 / 2\right)$

Intensity through three polarizers: unpolarized incident light of intensity $I_0$ becomes $\frac{I_0}{2}$ after the first polarizer due to averaging.

Light then polarized at $30^{\circ}$ relative to the second polarizer's axis transmits with intensity $I_2 = \frac{I_0}{2} \cos^2(30^{\circ}) = \frac{3I_0}{8}$.

The third polarizer is at $90^{\circ}$ to the first, hence $60^{\circ}$ to the second's axis, giving $I_3 = I_2 \cos^2(60^{\circ}) = \frac{3I_0}{8} \cdot \frac{1}{4} = \frac{3I_0}{32}$.

$\boxed{\frac{3I_0}{32}}$