In this paper, we report a three-dimensional tracking technique of small moving metallic targets in a cluttered environment using millimeter-wave Frequency-Modulated Continuous-Wave radars. Trajectories are determined from the estimation of the Direction of Arrival of signal backscattered from targets illuminated by the steerable radar beam. The electromagnetic clutter is mitigated thanks to an original image processing technique based on isolines, that is, lines in the radar images along which the echo level is the same. Our targeted application is the indoor detection at short range (up to 1.5 meters for a single radar) of tagged flying insects for behavioral studies.