Light travelling 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

As the ray moves toward the normal while entering medium 2 from 1, we have n₂> n₁
For total internal reflection at interface of 2 and 3, n₂ > n₃.
Besides $n_3$should also be less than $n_1$or else ray would have
emerged in medium 3, parallel to its path in medium 1.
Hence, n₃ < n₁ < n₂ is the correct order.