The unstable dynamic evolution of the magnetotail in response to a sudden occurrence or enhancement of resistivity is studied by means by a three-dimensional, time-dependent, nonlinear, resistive MHD code. We start from a realistic three-dimensional self-consistent tail model, including flaring in y and z and plasma sheet thickening toward the flanks. Many features that are believed to characterize the expansion phase of substorms developed without any driving mechanism at the boundary. Confirming earlier two-dimensional results, we found plasma sheet thinning, occurrence of strong flow and negative Bz, induced electric fields and formation of neutral lines, and a tailward-moving 'magnetic bubble' or 'plasmoid'. In addition, the results show that the occurrence of Bz<0 and the shape of the neutral lines are more complex than in 2-D models, with the negative Bz generally more restricted in time and space than strong tailward flow. The increase in plasma sheet thickness and Bz toward the flanks of the tail causes the reconnection and acceleration process to be limited in the y direction. Earthward from the main x line, a diversion of the cross-tail current is found, being earthward on the dawn side of the tail and tailward on the dusk side. Although the magnetic field turns so as to stay nearly perpendicular to the diverted current, net field-aligned components of the current remain inside the plasma sheet. These have the signatures of region 2 currents (Iijima and Potemra, 1976) flowing tailward on the dawn side and earthward on the dusk side.