Due to the merit of low cost and flexible flight, small drones are showing tremendous application prospect in both military and civil fields. Aiming at public area monitoring, a synthetic aperture radar (SAR) imaging model using drone fleet borne radars is proposed. In this model, the radars illuminate the imaging area while the drones fly forward straightly. To make the implementation easier, we assume that each radar transmits signal and receives echo independently. Furthermore, the frequency-division (FD) signals are adopted for different radars to avoid interference. With broadband signal, synthetic aperture via the motion of the drones and linear array formed by the multiple radars, three-dimensional (3D) imaging can be realized. Numerical simulations are conducted to verify the effectiveness of the proposed model and the 3D image formation method.