Anthropomorphic hydraulic manipulator arms are extensively utilized for moving heavy loads in many industrial domains, e.g., in off-shore, construction and mining. By equipping these manipulator arms with an additional 3 degrees-of-freedom (DOF) spherical wrist mechanism, the dexterity and working envelope of the manipulator can be increased. For versatile operations, the motion range for the hydraulic wrist actuators should be close to 360 degrees with high torque output, with a compact volumetric size. Although the wrist can provide a high power-to-weight ratio, the actuators also introduce significant non-linearities in their dynamic behaviors. The complex dynamic behavior combined with high loads yield significant challenges in closed-loop control design. In this paper, we design a novel subsystem-dynamics-based controller for a hydraulically actuated spherical wrist mechanism utilizing the virtual decomposition control (VDC) approach. The proposed 3-DOF wrist controller is designed to be modular; thus, it can be connected as an plug-and-play subsystem into our previously designed state-of-the-art controller for a 3-DOF hydraulic manipulator arm. Stability proof of the overall 6-DOF system is provided. Experiments with a full-scale commercial hydraulic manipulator arm equipped with the 3-DOF spherical wrist demonstrate the effectiveness of the proposed method.