V3 could show just about anything.
V1 shows the input voltage, which is what comes from the AC supply.
V2 shows the output voltage, which is approximately the transformer ratio times the AC supply.
However, there is no return path for V3, because the transformer isolates. In practice, stray capacitance, details of transformer construction and impedance of the voltmeter would all affect the reading of V3. There would be next to no current flow if V3 were replaced with an ammeter. That would not be the case with V1 or V2.
If one of the connections on the secondary of the transformer were connected to one side of the primary, then the voltage on V3 would be defined. For instance, if the two connections on the transformer with dots were connected, you would get V3 = V1-V2.