1. A method for determining a point-of-gaze (POG) of a user in three dimensions, the method comprising: presenting a three-dimensional scene to the user; capturing image data which includes images of both eyes of the user using a single image capturing device; estimating a first line-of-sight (LOS) vector in a three-dimensional coordinate system for a first of the user's eyes based on the image data captured by the single image capturing device; estimating a second LOS vector in the three-dimensional coordinate system for a second of the user's eyes based on the image data captured by the single image capturing device; determining the three-dimensional POG of the user in the scene in the three-dimensional coordinate system using the first and second LOS vectors as estimated based on the image data captured by the single image capturing device; wherein determining the three-dimensional POG in the three-dimensional coordinate system comprises: determining a third vector in the three-dimensional coordinate system which intersects the first and second LOS vectors and is orthogonal to both the first and second LOS vectors; estimating the three-dimensional POG to be the midpoint of the third vector.