1. A three-dimensional image display method in a three-dimensional image display system, the method comprising: generating a mask image corresponding to an object image; generating a background image such that a shadow of the mask image is disposed on the background image; two-dimensionally displaying the object image; three-dimensionally displaying the background image via a lens array; and combining the two-dimensional object image and the three-dimensional background image such that the object image is disposed on the shadow of the background image, wherein, in combining the object image and the background image, one of the object image and background image is formed as a real image and the other is formed as a virtual image, by using a half mirror, wherein the half mirror is configured to transmit one of the object image and background image and reflect the other, and wherein the generating of the background image comprises: disposing the background image between a lens array and the mask image so that the background image overlaps with the mask image; and projecting the mask image and the first background image onto an input panel via the lens array so that the shadow of the mask image is projected on the background image.