[HayZee518]

what you can do is, shut off the breaker for the circuit. take some wire and strip off both ends. fasten one set of these [white-black]to your gfci. on the other end strip off some insulation strip off some insulation on your [white-black] going to the X-10. Now in the gfci box, splice all three whites - put on a wirenut and the three blacks, put on a wirenut and fold this back into the box and replace the gfci. In the other box [three gang] you'll have a hot and neutral for the X-10. the wires from the three way, either tape them off or put one wirenut on each wire and shove the wires in the box.