There are typically 6-8 power terminals on the transformer depending on internal configuration. In North America H1, H2, H3 correspond to high voltage phases A, B, C. H0 would be high voltage neutral if applicable. X1, X2, X3 would be low voltage phases A, B, C and X0 the low volyage neutral.
If your adding a transformer a knowledgable person should be involved to ensure phasing of other transformers is the same and transformer configuration is appropriate for your type of system.