[Herald1360]

The low secondary volts in single phase mode suggests that the transformer primary was delta connected giving a primary voltage of 415V.

Putting 240V on such a primary will give a lower secondary voltage than you might expect.

In any case a 3-phase bridge rectifier gives a dc output close to the peak L-L secondary voltage with only about 4% ripple so again, secondary voltages may not be what you would expect.

My three phase transformer theory is a bit sketchy- I only had to design power supplies using them to drive three phase bridges for the raw dc rails. Someone like Ed Dinning should be able to do chapter and verse for you if necessary ......