Three Mirrors

Three mirror lines make a triangle.
For each corner we are free to choose the rotational symmetry:

center    (the corner at the left)
left  (as seen from the center)
right  (as seen from the center)

sum of angles = o

If the sum of the corner angles is not 180o degrees, then one of the mirror lines has to be a circle arc!

What is the mirror image at a circle arc?