This work addresses the development of a unified control strategy, based on nonlinear control techniques, for hybrid tri-tiltrotor UAVs, so that a simple trajectory is followed. A model of a tri-tiltrotor UAV is derived, detailing the forces and moments that act on the system, followed by a unified control approach that considers the system dynamics as a whole. Backstepping control and nonlinear optimisation are used for position and attitude control to calculate force and moment references and a control allocation strategy based on nonlinear optimisation is proposed. Two trajectories are defined; in the first trajectory, the UAV is expected to fully transition from rotary-wing to fixed-wing configuration, while in the second, it is expected to fly in an intermediate configuration. To validate the approach, simulations for these trajectories are performed and the results are analysed.