When the light enters from optically rarer to optically denser medium, it bends towards the normal. While travelling from ${\mathit{n}}_1$ to ${\mathit{n}}_2$ light bends towards the normal, so, ${\mathit{n}}_2>{\mathit{n}}_1$. Similarly, ${\mathit{n}}_3>{\mathit{n}}_2$. Therefore, ${\mathit{n}}_3>{\mathit{n}}_1$.