The traditional hardware phase lock loop cannot operate properly in harmonic distorting. Also it can only extract the phase angle of a single phase and cannot extract the positive sequence phase angle of a three-phase power system. In order to adapt to the three-phase power system， the traditional single-output variable step size least mean square（VSS-LMS） algorithm is extended to multi-output system to construct a three-phase magnitude-phase-locked loop（3-MPLL）. The proposed 3-MPLL can automatically adapt to the harmonic distorting of three-phase power system by adjusting step size accordingly， and can enhance the convergence speed greatly by resetting the step size to a large value when dips occur. Experiment results show that the 3-MPLL is not only available in the harmonic and three-phase unbalanced condition， but also can detect the magnitude and phase angle change of the positive sequence voltage of three-phase system with little time delay，thus it can be widely used in the power electronics equipments connected to the three-phase power system.