For three way or more dimming, for low cost I use the pre-set dimmers, they have a three way rocker switch at the bottom with a slide dimmer, it will have two of the same color wires which are the travelers, and a wire of a different color which is the common. the dimmer can go at either 3-way location, load, or line, and what ever the dimmer is set for the other switch's will just turn on or off the light at this set level, multi-location dimmers will use one of the travelers as a LV signal path, and you can control the dimming from any location that has one of the matched dimmers, for two point dimming there are dimmers out there that have the dimmer control in the traveler wire, and if you put one of these at each 3-way and wire them so one has the dimmer in traveler wire 1, and the other is in traveler wire 2, they will dim from two locations.