Assuming batteries have the same voltage and current rating, the more power available, the more power the bulb can draw from the battery since the power in a battery-powered circuit is proportional to the number of batteries used. So, the circuit with three batteries would produce the brightest light bulb.
During the "U" part of the turn, the car would follow an approximately circular path, and if it's moving at a constant speed, it would have to accelerate toward the center of the circle in order to change its direction.
