While I agree with the other answer that often there are several single phase supplies inside, connected to separate phases, note that there are some commercial solutions if you truly have one big power supply with a three phase primary on the transformer. See http://www.phaseconverter.com/ and http://www.phase-a-matic.com/ for example. These devices have a long life time and can be found on industrial surplus markets (which of course are on the web too)