A new achromatic isochronous deflection system is described. It consists of three magnets: two small anti – symmetric dipoles and one dipole with over 90° bending angle. The achromatic and isochronous relations for the system are obtained by six dimension transport matrix method. The system is characterized by excellent achromatic and isochronous properties and compact configuration. The three ?magnet bending system is suitable for linac beam deflection of free ?electron laser facility.