The effects of the three-axis magnetic field on the performance of the SERF co-magnetometer are studied theoretically and experimentally in this paper. We have established the co-magnetometer’s complete steady-state response model to the three-axis magnetic field. Based on the established model, we experimentally measure the effects of the three-axis magnetic field on the scale factor, as well as the direct magnetic field errors and the coupling errors. The experimental results are consistent with the theoretical model. The results indicate that the z-axis magnetic field has the greatest impact on the scale factor, while the x-axis magnetic field brings the largest direct magnetic field error. The three-axis magnetic field introduces and amplifies other coupling errors to varying degrees. This work is crucial to our understanding of how magnetic fields affect the co-magnetometer’s performance and how to reduce errors caused by magnetic fields.