Magnetic hyperthermia ablates tumor cells by the heat dissipated from magnetic nanoparticles (MNPs) exposed to an alternating magnetic field. The applied magnetic field plays an important role in magnetic hyperthermia, since both its amplitude and uniformity directly affect the treatment results. The amplitude of magnetic field can be easily set to a required value for reaching a desired maximum temperature, but the uniformity requires a modification in the experimental setup. Although in practice the magnetic field is not homogeneous, most of the previous reports generally assume it to be. In this study, the homogeneity of the magnetic field is improved by adding a third coil between the original two in the conventional Helmholtz coils. In addition, a differential evolution algorithm is used to optimize the parameters for the improved coils by considering a proposed objective function. The results of applying the proposed method are evaluated considering a numerical simulation model of magnetic hyperthermia and compared with the traditional approach using the effective volume percentage of therapeutic temperature (EVPTT) as performance index. The results demonstrate that the uniformity in the magnetic field of the proposed method is significantly better than in the case without considering the proposed optimization. In terms of therapeutic efficiency, the improved magnetic field will translate as better temperature distribution in the tumor region, but the gains are much more evident when the tumor is away from the coil center.