No, you can't put a plain X10 switch in with the 3-way ciruit. If the wiring is as I think it is your choices are as follows:

1. Run and extra romex to that last switch location
2. Replace that last switch location with a keypad
3. Take thet last switch location out of the circuit

In-wall keypad transmitters start as low as $21.07. Look at the XPT1 at [Link: homeautomationnet.com]

To trace the wires you'd turn off all the power then use the multimeter to determine which romex goes to each box by shorting one romex's black and white wire (make sure the power is off) then looking for the shorted wires in the other boxes.