An effective spin model for Mott insulators is determined by the symmetries involved among magnetic sites, electron fillings, and their interactions. Such a spin Hamiltonian offers insight to mechanisms of magnetic orders and magnetic anisotropy beyond the Heisenberg model. For a spin moment S bigger than 1/2, single-ion anisotropy is in principle allowed. However, for d 3 Mott insulators with large cubic crystal field splitting, the single-ion anisotropy is absent within the LS coupling, despite S=3/2 local moment. On the other hand, preferred magnetic moment directions in d 3 materials have been reported which calls for a further theoretical investigation. Here we derive the single-ion anisotropy interaction using the strong-coupling perturbation theory. The cubic crystal field splitting including eg orbtals, trigonal distortions, Hund's coupling, and spin-orbit coupling beyond the LS scheme are taken into account. For compressed distortion, the spin-orbit coupling at magnetic sites favors the easy-plane while that of anions leads to easy-axis anisotropy. We apply the theory on CrX3 with X = Cl and I, and show the dependence of the single-ion anisotropy on the ratio of the spin-orbit couplings of magnetic and anion sites. Significance of the single-ion anisotropy in ideal two-dimensional magnets is also discussed.