The physical design of the transformer is different from a signal phase transformer, bad things happen when you just feed one leg! Don’t know what would occur if you ran just the same line to all three primaries in parallel but can’t see it being a good thing.

I have had lost one phase before on transformers at work and they get hot and loud fast. Also, weird voltages appear on the missing leg. Think you would be happier just replacing the transformer outright with a signal phase transformer.

Ray F/KA3EKH