This paper analyzes the magneto-optic (MO) effects in three layer surface plasmon polariton (SPP) slab waveguides as a function of the mid layer thickness in a transversal configuration, while the applied magnetic field is parallel to the interfaces and normal to the wave propagation direction. Two different configurations of insulator–metal–insulator (IMI) and metal–insulator–metal (MIM) with ferromagnetic dielectric or metal layers are analyzed. We have derived exact dispersion relation by considering MO effects for all three layers. We have shown that the SPP waves in these anisotropic waveguides exhibit nonreciprocity with respect to their direction of propagation. The propagation constant and length depend on the configuration and the thickness of the mid layer, which waveguides with maximum nonreciprocity and minimum loss can be obtained. These configurations could be used to design active elements, such as isolators and circulators in photonic integrated circuits (PICs).