Calculate the current through the network and show that the potentials at A,B and C. Since it is a series network, add the sources and resistor to get a single source and resistor. Using I=V/R find the current. The voltage drop across the resistor from each source is found to be equal to the voltage source itself. Thus since all the resistors are equal the points A,B and C will be of same potential. Verify with the voltmeter reading in the experiment.