Three-dimensional MHD simulations of the Kelvin-Helmholtz instability (KHI) have been performed to investigate its relevance to the magnetotail-flank situation. The effect of the KH-stable lobe region on its growth at the plasma sheet--magnetosheath interface can be a crucial factor. To assess this effect, we study how the KHI grows in an unstable layer of finite thickness (plasma sheet) sandwiched between stable regions (north and south lobes). The results show that when the magnetosheath magnetic field is northward, the instability grows vigorously to form a highly rolled-up vortex even when the plasma sheet thickness is smaller than the wavelength of the fastest-growing mode. Furthermore, two rolled-up vortices can coalesce into a larger one as long as the thickness is larger than the wavelength. The coalescence under the tail-flank geometry causes strong stretching of the field lines, leading to the condition under which reconnection could easily be triggered. These findings suggest that when the magnetosheath condition is favorable for the KHI and when the plasma sheet is thicker than a few RE, the vortices become an important element of the tail-flank dynamics. We also found that in rolled-up vortices, the tailward speed of the plasma sheet plasma exceeds that of the magnetosheath flow in regions where it penetrates toward the magnetosheath. Since this overshoot acceleration is seen only when the vortex is rolled up and is detectable even from single-spacecraft observations, we suggest that it can be used as a marker of the roll-up of vortices.