1. A method to synthesize additional information in a three-dimensional (3D) image, the method comprising: determining, by a processor, a disparity value of the additional information to be added, wherein the disparity value of the additional information is greater by a predetermined value than a disparity value of an object to be located behind the additional information; and displaying the 3D image where the additional information is overlaid according to the disparity value of the additional information, wherein the additional information comprises at least one informative element distinguished from the 3D image.