We consider, within the Hubbard model, nonequilibrium transport through a triangular molecule [or, equivalently, triple quantum dot system (3QD)], weakly coupled to electrodes. The exact solution of the eigenproblem for the isolated 3QD is used for computation of the current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of the device in the sequential tunneling limit. We analyze the effects of symmetry perturbation of the device and show that local energy perturbation at one of the sites coupled to the electrodes can lead to a negative differential resistance as well as rectifying properties of the device.