Please allow me to ask another question - How many screws are on the rocker switch that you want to replace with the slide dimmer? I am talking about the screws that have wires connected to them, and we can ignore the ground wire. Also, it would help me answer your question if you can tell me what colors the screws on the switch are and what color wires are attached to the screws. Thank you for your patience as we work through this.
Thank you. A switch with 3 wires connected to it is a 3-way switch, meaning there should be a second switch somehwere nearby that will operate the same set of lights. You will need to use a 3-way dimmer switch to replace the existing switch. The 3-way dimmer will have 4 wires coming out of it, with one of them being the ground wire. Of the other 3, there will be 2 that are both the same color and a third that is a different color.
If you look at the existing switch, there should be 2 screws that are the same color (probably gold or brass) and a third screw that is a different color (probably black). Please make note of how the wires are connected to the screws on the existing switch and connect the new 3-way dimmer in much the same way - by connecting the 2 wires that are connected to the 2 screws that are the same color on the existing switch to the 2 leads from the 3-way dimmer that are the same color. This leaves one more wire from the wall and one more short lead from the 3-way dimmer, simply connect these last 2 wires to each other and you should be good to go.