I installed dimmers on 3way switches at our old house. It took me a while and a phone call or two to an electrician buddy, but they worked great. The only issue I had was that my wife would not push the switch off to the point where it clicked, therefore leaving it on just a little, when she did this, the other switch was disabled, so you had to walk all the way to the other side of the room and turn the switch all the way off, then back across a dark room to the switch you wanted to use. Sounds screwy, but one switch has to be off for the other one to dim or work at all.