According to given question,
     $I_1:I_2=3:1, a_1:a_2=?$
The intensity of light due to an illuminated slit is directly
proportional to the width of the slit, if $\omega$₁ and $\omega$₂ be the width of two silts and I₁ and I₂ are the intensities of light on the screen due to the respective slits, then

$\frac{w_1}{w_2}=\frac{I_1}{I_2}={\left(\frac{a_1}{a_2}\right)}^2$

where, a₁ and a₂ are the amplitudes of the corresponding light wave.
Here,  $\frac{w_1}{w_2}=\frac{3}{1}$

$\Rightarrow {\left(\frac{a_1}{a_2}\right)}^2=\frac{3}{1} \text{ or } a_1:a_2=\sqrt{3}:1$