To get three images of a single object, the angle between the two plane mirrors should be

let the angle between mirrors $=\theta$

n = 3

$3=\frac{360}{\theta }-1$ $\text{if}\frac{360}{\theta }\text{is}\text{even}$

$\theta =\text{90°}$

When we get 3 images for symmetrical or as symmetrical position of object

$3=\frac{360}{\theta }\text{if}\frac{360}{\theta }\text{is}\text{odd}$

When $\theta =\text{120°}$ if object is kept symmetrically then only we get 3 images