The dq- frame impedance model is increasingly employed to analyze the grid-converter interactions in three-phase systems. As the impedance model is derived at a specific operating point, it is required to connect the converter to actual power grids during the impedance measurement. Yet, the nonzero grid impedance causes cross-couplings between perturbation and response signals, which consequently jeopardize the accuracy of impedance measurement. This article analyzes first the coupling effect of the grid impedance on the measured impedance, and then proposes a multiple-input multiple-output parametric impedance identification method for mitigating the effect. Instead of using the fast Fourier transform, the method allows for obtaining the parametric impedance model directly from the time-domain data. Further, with the simultaneous wideband excitations, only a single measurement cycle is needed. The effectiveness of the method is verified in both simulations and experimental tests.