This paper presents an on-line three dimensional trajectory planner for an unmanned vehicle following a Leader vehicle, where the Follower remains at a specified relative position with respect to the Leader. The planner defines the trajectory for a virtual vehicle that will be used as reference for the real Follower vehicle. The virtual Follower behaves like a three dimensional trailer attached to the Leader and its reference frame is used to specify the relative position vector. At the kinematic level, the proposed planner requires only the knowledge of the Leader's linear speed, but not its angular speed, and the Follower's trajectory can be generated on-line, in the sense that no a priori knowledge of the Leader's trajectory is required. Experimental results obtained with quadrotor vehicles are presented, which demonstrate the richness of the planned trajectories.