Good Evening,
I would consider either a hard wired or a commercial RF solution. You would have to do phase coupling on the transformer secondary to get the signal on each of the three phases. That is on one transformer then you have to get the signal to the secondary(s) of the other transformers. X10 does not typically go through power a transformer, which is why you and the neighbor down the street do not interfere with each other.
If you have networking installed this could make the problem much simpler. The typical Ethernet uses only two of the four pairs in the cable. Pins 1,2,3 & 6 are used the others are not. Pins 1&2 are one pair pins 3&6 are the second pair (yes they split the pair). Be sure IT lets you do this, hooking up wrong can destroy some expensive equipment.
Have Fun
Gil Shultz