<p>Existing robotic hands with dozens of motors can grasp various objects through dexterous motion, but they are difficult to fabricate and control, and have relatively low durability. By contrast, a two-finger gripper can grasp objects by using a single motor and simple control algorithms, but it cannot grasp long or spherical objects. The grippers used in service fields should preferably have the elements of a single motor, simple control, and the ability to grasp a variety of objects. In this study, we propose an adaptive three-finger gripper with an underactuated mechanism that can grasp objects with complex shapes by using a single motor and simple control commands. The proposed gripper is designed to stably grasp objects of complex shapes with its fingers that use magnetic-gear-based belt drive mechanism and passive joints. A series of experiments showed that adaptive grasping of objects having complex shapes is possible, and even long objects can be grasped firmly.</p>