I always thought/assumed that brushed motors had two brushes in them. Mine has three. How does it work?
LOL. That was a long wait for the punchline.
It's not the number of balls that matters, it's the size !!!
With CaptainTuna's comment, do you mean three brushes, three wires, or three of something else?
A windscreen wiper motor doesn't always just go round and round. If it's a third wire you're wondering about, it could be a "trigger" of some sort that causes the motor to do just a specific run. Check this little guy for an example.
Check the link below. It explains that most DC motors have 3 poles so the rotor doesn't get stuck when positioned exactly between the magnets.