1. A method for providing a three-dimensional (3D) image, the method comprising: acquiring a plurality of first projection images which are created by detecting, by using an X-ray detector, X-rays which are emitted from an X-ray source toward an object; generating a 3D volume which represents the object by using the plurality of first projection images; creating a plurality of second projection images by applying a forward projection to the 3D volume; and displaying the plurality of second projection images by using a 3D display.