The increasing popularity of different kinds of 3D displays such as stereoscopic 3D monitors, auto-stereoscopic displays, and head mounted displays (HMDs) has led to visual discomfort being considered as one of the major concerns in the 3D industry. Previous research studies on visual discomfort have considered various disparities and motions in 3D videos to identify vergence-accommodation linkage conflicts. In this paper, we measure visual discomfort occurring from viewing 3D videos with various contrast changes using perceived symptoms questionnaires, measured near point of convergence, eye blink rate, and saccadic movements. We compare visual discomfort for different displays such as stereoscopic 3D (s3D) display, auto-stereoscopic display, and HMD. From our experimental results, we conclude that visual discomfort increases when the subject is watching 3D videos using auto-stereoscopic displays. In addition, brighter videos caused more visual discomfort compared to darker videos.