In this paper, we consider formation control of fully- actuated marine craft systems with motion in three degrees of freedom (3-DOF). In particular, we consider the case where n independent vehicles are expected to satisfy a given formation pattern and where the formation as a whole is expected to track a smooth reference trajectory. To this end, we propose decentralized control laws for each vehicle and show that the formation error dynamics is globally exponentially stable under closed- loop control. The control laws are designed in the framework of Lyapunov direct method and graph theory, and the independent control laws are designed to provide feedback from any adjacent vehicle.