Visual servoing control of robot manipulators represents a natural option in unstructured environments and it is specially attractive when the task to achieve can be expressed directly in image coordinates. In the present work a control law is developed for trajectory tracking of three degrees of freedom robot manipulators. The main characteristic of the proposed algorithm is that it does not need any dynamic model of the system and image coordinates are employed directly for feedback, while an observer is designed for velocity estimation. To carry out 3D movements no stereoscopic vision is used. Rather, a simpler configuration employing two fixed cameras is proposed which makes unnecessary their calibration and helps avoid occlusions for most of the robot's workspace. The developed theory was successfully tested in an industrial manipulator.