You need three 1.5V batteries and a 3V bulb for this.

Connect the ends of 1 battery to the bulbholder using two wires. The bulb should light dimly. This is because we do not have enough batteries to match the bulb.

Now place two batteries in line and facing the same way, and connect these to the bulb. The bulb should now light brightly. We now have enough batteries to match the bulb.

If we had placed three batteries in a row, all facing the same way, the bulb would light even more brightly, but it would not be long before it burned out. So it's not worth trying this!

Hint: when you want to see the effect of changing batteries on the way a bulb lights, it is always best to work under what you think the number of batteries should be.