if you going to use a regular incandescent put the dimmer AFTER the 3
way switches in the line feeding the lamp.

this way you dont have to disturb the 3 way switches they are easy to
muck up and persumably you wouldnt be changing the lamp britness
often so the dimmer can go in a easy place to install.

lamp life will skyrocket.

although energy wise your probably better off with CF