  We calculate the conductance through a system of three quantum dots 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 quantum dot,
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".
