We developed a three-dimensional motion tracking system that displays trajectories as augmented reality in real time on a tablet device. The features of this system are as follows: (1) the three-dimensional trajectory can be observed from any direction. (2) Velocity and acceleration can be visualized as augmented reality. The device consists of an Intel D435, a spatial recognition camera, and a Raspberry Pi 4 for data processing and transferring. The data are broadcasted to tablet computers, allowing simultaneous observation from multiple tablet devices. The sampling rate of the detection is 8 fps with a measurement error of 1.2%. Since even a general-purpose tablet device can be used to visualize trajectories as augmented reality with this motion-tracking system, it can be widely applied in the study of dynamics in physics from the junior high school to the university level.