Hyper-Rayleigh scattering experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of three-arm merocyanine derivatives. They exhibit an octupolar hyperpolarizability response with lower amplitude than crystal violet due to a lower extent of the photoinduced charge transfer and reduced bond length alternation. Strong effects on the second-order optical response measured close to the two-photon absorption level are clearly evidenced; for example, the effective measured polarization ratio deviates below the ideal octupolar value of 3/2 even at very low excitation power. These effects are attributed to two-photon absorption resonance, which we believe modifies dynamically the population of the ground state versus that of the excited state.