Couple of things.
Firstly, if you're going to have wide open, some roll-off, and a lot of roll-off, then I would do it with two different capacitors rather than resistors and capacitors. The "dull tone" capacitor would probably be a 0.022uF capacitor, but for the "medium tone" position you might want to look at something like a 0.01uF or even a 0.0047uF capacitor.
You can use a DPDT on/on/on switch to achieve this result. Look at this diagram:
The white wire marked "output to volume" would be connected to the place on the volume control that you currently connect the tone control to (probably the left lug).
The wire marked "Hot A" would not be there - "A" would be your "wide open" position.
The wire marked "Hot B" would be replaced with your "medium" tone capacitor, with the other end connected to ground.
The wire marked "Hot C" would be replaced with your "dull" tone capacitor, again with the other end connected to ground.
Hope that helps.