For most single phase transformers, polarity is unimportant, all that matters is that the turns ratio must be correct for the intended application (plus of course suitable power rating and design)
For a 3 phase transformer intended to work on its own, polarity is umimportant.
For a 3 phase transformer to work in parralell with one or more other transformers, then polarity is important. Transformers of differing polarity wont work in parralell, no matter what alterations may be made to the external connections.
A 3 phase transformer intended to supply utilisation voltages normally has only 4 output terminals, one for each phase, and a fourth terminal for the neutral, intended to supply 3 phase, 4 wire.
There will be 3 windings inside the transformer, with one end of each winding connected together internaly to give the neutral.
If two otherwise identical transformers are manufactured, one with the bottom ends of each winding connected together, and the other one with the top ends connected together, then these transformers are said to have opposite polarity.
Each of them will work just fine on its own and provide a standard 3 phase, 4 wire service, but they will never work together.
It would be possible to build a 3 phase transformer with each end of each low voltage winding connected to a terminal, 6 terminals in total.
Such a transformer could be connected with either polarity, by altering the external connections.
This is not usual in practice as it adds to costs.
All British Standard transformers are built with the same polarity, and I would presume that similar standards exist in other countries.