We present a new optical design of an off-axis three-mirror system (OTS) that is corrected for spherical, coma, astigmatism, and field curvature aberrations. Our design methodology begins using paraxial theory and continues with the third-order approach for a coaxial three-mirror system (CTS) with a wide field of view and high resolution. We propose to use only two optical elements because the primary and tertiary mirrors will be integrated in a single optical element, the secondary mirror will be the other optical element. For our final design, we will use only off-axis surfaces. Thus it will be possible to obtain a compact, light, easy-to-align optical system with high stability. However, when the CTS is transformed into an OTS, new aberrations are generated. The last step in our methodology is to use freeform surfaces to compensate for new aberrations. With this methodology, a limited diffraction system is obtained.