The magnetic sensor of smartphone with Phyphox App was used to build an experimental device for the three-body coupled pendulum by measuring the change of magnetic induction intensity． The beats and energy transfer among pendulums can be clearly observed． By changing four initial conditions，different oscillating modes occur．In addition，with increasing coupling distance and the beat periods of mid pendulum monotonously decrease． The three-body coupling problem was solved by Maple software． It is found that the solution of equations is basically consistent with the experimental results．The fabricated experimental device is simple and suitable for home experiment．