A 3 way dimmer switch? Is there really such a thing? I'm not an electrician, nor am I being facetious. I just never heard of a 3 way dimmer.

New blacks to the wires that the two red leads went to and the single red lead to where the black used to go to. Green wire to an equipment ground if available in the box.

Love three-way dimmers. Great for things like hallway lights, that are nice to keep on dim at night. Readily available. You probably want the push-pull/rotary type, because the slide type won't work properly in a three-way circuit.