Don't know what this means. It's a single-phase transformer. The primary may be connected phase-phase, but what's the purpose of the center tap?
In the shown circuit, there's no "return voltage", unless the load feeds back a voltage between L2 and L3. But you don't show a load, what is it? Show the complete schematic.
Generally, it's unusual to transform a three-phase supply with only two single-phase transformers, because it creates an unbalanced voltage drop.