The hybrid implicit-explicit finite-difference time-domain (HIE-FDTD) scheme for a 2-D transverse electric (TE) wave is extended to a full 3-D electromagnetic wave in this paper. With the weakly conditional stability, this approach simulates shielding effectiveness of an enclosure with high computation efficiency. Numerical formulations of the 3-D HIE-FDTD scheme are presented, and simulation results are compared to those obtained by using the conventional 3-D FDTD and alternating-direction implicit (ADI) FDTD methods. The accuracy and efficiency of the 3-D HIE-FDTD for prediction of shielding effectiveness are validated by numerical simulation results.