Three-phase three-wire connection means that there is no neutral point grounding, and there are star and triangle connection modes. No matter which connection mode, if it is symmetrical, the three voltages are symmetrical, if it is asymmetrical, it is asymmetrical. For star symmetry, if you want phase voltage, you need to connect a voltmeter to A, B, B, C, C and A. For triangle symmetry, the lines are the same. For asymmetry, three voltmeters need to be connected. The voltage transformer you mentioned can't measure a lot of voltage, and it is fixed on the secondary side. It is 1 00V, and the wiring of1secondary side can be connected as shown in the figure. You are equivalent to using the voltage of each phase as the power supply voltage of the transformer. I study electricity, please forgive me for my shortcomings.