We calculate the conductance through a system of three quantum dots (QDs) under two different sets of conditions that lead to spin filtering effects under an applied magnetic field. In one of them, a spin is localized in one QD, as proposed by Delgado et al. [Phys. Rev. Lett. 101, 226810 (2008)]. In the other one, all dots are equivalent by symmetry and the system is subject to a Rashba spin–orbit coupling. We solve the problem using a simple effective Hamiltonian for the low-energy subspace, improving the accuracy of previous results. We obtain that correlation effects related to the Kondo physics play a minor role for parameters estimated previously and high enough magnetic field. Both systems lead to a magnetic field tunable “spin valve”.