Use to think that the first notch opened a flap and the other 2 were Hi and Low fan speeds, but someone could perhaps chime in to confirm or correct.
I do know the blower is one speed with a resistor that reduces the current to about half speed.
Low speed is power through the resistor and high is power around the resistor directly to the brushes.
Speeds are half and full which explaining the weak air-flow.
Sometimes the switching arm can get bent and prevent contact as well.