This paper proposes a multi-antenna approach of a synthetic aperture radar method for 3D UHF-RFID tag localization by exploiting an Unmanned Grounded Vehicle (UGV). The UGV is remote-controlled to move inside a complex indoor environment, and the knowledge of the reader antenna trajectories is achieved with millimeter accuracy through a commercial motion tracking system. By processing the tag backscattered signal phase data collected from two RFID reader antennas, coherent and non-coherent combining processing are performed to improve the estimation of the 3D tag position with respect to the case of a single antenna. Differently from other localization techniques, neither reference tags (anchor tags), nor large phased array antennas are required.