Depends if the phases are joined at the electrical consumer unit or not. The Powerline network will run everywhere where there is a connectivity path. If the circuits are isolated, then the network can be joined together using CAT5 cables. A phase coupler will also work.
Within commercial buildings, 3 phase power is common. Power Ethernet has written a whitepaper on the subject of implementing Powerline networking in large installations, you can fill the for at Contact and we will send it to you.
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