In order to understand the interaction between solar wind irregularities and the closed magnetosphere, three-dimensional (3-D) computer simulations for ideal magnetohydrodynamical (MHD) equations are performed. This paper considers only the situation in which the initial interstellar magnetic field is either parallel or antiparallel to the magnetospheric magnetic field. The irregularity is modeled as a ball, which initially moves toward the magnetosphere. The penetration of the irregularity in the 3-D model is found to be less efficient than in a two-dimensional (2-D) model. Through a comparison of the present results from 3-D simulations, not much difference has been found in the efficiency for the irregularity to penetrate into the magnetosphere between two situations in which the interstellar field is parallel or antiparallel to the magnetospheric field.