Light traveling through three transparent substances follows the path shown in figure. Arrange the indices of refraction in order from smallest to largest. Note that total internal reflection does occur on the bottom surface of medium 2

At the upper surface , critical angle is given by sin $r_c=\frac{n_1}{n_2}$ and at the lower surface critical angle is given by $\sin {r_c}^{\text{'}}=\frac{n_3}{n_2}$.In the given diagram, angle of incidence at the lower surface is greater than ${r_c}^{\text{'}}$which must be less$\frac{n_3}{n_2}<\frac{n_1}{n_2} \Rightarrow n_3<n_1$ .

Also from the given diagram $n_2>n_1.$