Off-axis optical systems have several important advantages over on-axis ones. However, high polarization aberrations, which play important roles in many applications, become critical disadvantages of off-axis systems. Thanks to the seven free design parameters, three-mirror reflective systems have a good potential to achieve low polarization. A general method to design low polarization off-axis three-mirror reflective optical systems is proposed in this paper. Based on genetic algorithms, several off-axis three-mirror systems with both low polarization aberrations and good wave aberrations are designed. The method proposed in this paper is versatile and can be used to design other types of optical systems that demand low polarization aberrations.