The answer is 1cm. the reason is that the distance of an image from the mirror is the same as the body from the mirror. So if at first ,the body was 3cm away from the mirror then the image is 3 cm from the mirror so if the mirror is moved 1 com towards the body, then the distance now is 2cm for both body and image. The distance of the image(2cm) from the first(3cm) is 1 cm I think.