I want to have a switch determine whether a battery pack gives 3, 4.5, or 6 volts.?

Hey, I'm working on a device that launches ping-pong balls, and I don't know how precisely to do the wiring. I want to have a single switch have four settings. Off, low, medium, and high. Low will provide 3 volts (2 batteries worth), medium will provide 4.5 volts (3 batteries worth), and high will provide 6 volts(4 batteries worth). What is the best way to do this?