The two bearings on one end are the angular contact bearings. The single bearing on the other end is just a standard ball bearing. This design is meant to trap the ball screw at one end by the two bearings to prevent any linear movement. This linear movement contributes to backlash. The other end is held in the radial direction but able to float in the linear direction to take up movement of expansion and contraction when the temperature changes. You don't want to lock the screw (in the linear direction) on both ends.
Normally the end with the two bearings is the end where you will attach the motor to power the screw. With this said it's most of the time at the top of the Z axis.