How can I use an oscilloscope to measure voltages that are referenced to nodes several hundred volts apart?
Since it is AC consider using a small transformer for each phase.  They can be quite small and the turns ratio will determine what you will get on the secondaries.  The quality of the transformer will determine the accuracy of the measurement.  The nice part is the voltages to the scope can be as low as you want and the transformers can have one side of the secondaries connected to ground and your scope's common terminal.