There are different ways to wire 3 way switches, so It is difficult to answer the question without knowing more about the setup.
This site explains the possible variations.
http://www.homeimprovementweb.com/information/how-to/three-way-switch.htm

Bruce

Read the instructions.

your laundry room has 2 entrances?

if you have 2 3-way switches then that means that the power source is at one switches common and the other switches common is feeding the light

find out which switch is getting the 120v source and connect your new sensor switch there

use the other wire ((perhaps the red)) and wirenut it to the "common wire" from the other switch box

then cap the ends of the unused wire in both boxes

Does the sensor need a neutral?