The article proposes low-level control schemes for a group of quadrotors with highly nonlinear uncertain models in formation flights. The paper suggests control strategies for formation flights in a 3D space. The concepts of flat system and virtual formation play important roles in the current approach. We assume that accurate information needed to control a quadrotor in formation flights is available to the control system. Despite of system uncertainties the control law ensures that the error of the attitude subsystem relative to a required reference attitude converges exponentially to zero. Regarding the translational uncertain subsystem the solutions of the associated error dynamics are ultimately bounded and the ultimate boundedness of the error between the actual trajectory and the desired one in the flying formation can be guaranteed. The control scheme allows formation splitting and merging and formation switching in flight. Simulation results and discussions conclude the paper.