The bulbs in the parallel circuit (to the left) are much brighter than the bulbs in the series circuit (to the right)!
The parallel and series circuits handle current from the battery differently. In your parallel circuit, current from the positive terminal of the battery divides into three streams that flow separately through the three lamps. These three streams of current rejoin after leaving the lamps and then return to the negative terminal of the battery. The current picks up 6.3 volts during its trip through the battery and it loses that entire 6.3 volts in passing through any one of the lamps. In other words, the voltage drop through each lamp is 6.3 volts. Since voltage measures the energy per unit of charge in a current and voltage drop is a loss of energy per unit of charge in a current, the current is leaving behind quite a bit of energy each second in each of the lamps. The lamps glow brightly.
In your series circuit, current from the positive terminal of the battery passes sequentially through each lamp and then returns to the negative terminal of the battery. The current loses about 2.1 volts as it passes through each lamp, a much smaller voltage drop than occurs in the parallel circuit. This smaller voltage drop means that each unit of charge passing through the lamp leaves behind less energy. With the current leaving behind so little energy each second in each lamp, the lamps glow dimly.
Answered by Lou A. Bloomfield of the University of Virginia.