You cannot have a three phase primary and a single phase secondary. You can have a hybrid, such as a 120/240 volt three phase with one phase having a grounded center tap. But only the one winding with the center tap will make 120 volts available. The other windings will only give you 240 volts. Calculating the KVA on such an animal is no trivial task.
If you had a single phase, 600 volt primary, and a single phase, 120/240 volt secondary, then the calculated KVA would be (7) times (50) times (240) equals 84,000.