We performed two-dimensional (2D) and three-dimensional (3D) hybrid simulations in open boundary models to study the nonlinear mirror-mode structures driven by the temperature anisotropy (T-perpendicular to/ T-parallel to > 1) of protons in the magnetosheath. In the open systems, because of the propagation of EMIC waves, we obtain the clearer non-propagating mirror-mode structures. We analyzed the relation between the mirror instability and the magnetic peaks and dips observed in the magnetosheath. In the 2D open boundary model with low beta (beta(parallel to) less than or similar to 1), we obtain fine structures of the magnetic dips at the nonlinear stage. In the 3D model, on the other hand, the mirror instability makes the magnetic peak structures with the same parameters. The parametric analysis indicates that the magnetic peaks also arise in both 2D and 3D high beta cases (beta(parallel to) less than or similar to 1) as shown by the Cluster observations. In the high beta cases, the high mobility of the protons helps continuous coalescence of the diamagnetic currents inside the magnetic dips. The coalescence makes the magnetic dips larger and shallower. Between the large and shallow magnetic dips, the magnetic peaks appear in the high beta cases. In the 3D models, because degree of freedom increases in the perpendicular direction, the continuous coalescence can take place even in the low beta cases. Thus, the magnetic peaks appear in the 3D models in both cases.