Transformers

No, it is not.

Stan, did you mis-read the question?

Using three single phase transformers in place of a 3 phase unit has been done many times.

A 3 phase transformer is nothing more than 3 - 1 phase transfomers in one cabinet, connected in a Delta or Wye configuration.

Wow, I've never seen it done, and I figure it violates keeping all circuit conductors in the same raceway, etc, codes.

I have done quite a bit of commercial/industrial work over the years, but have yet to see this.
Good to know.

Ever see a cluster of three pole mounted kettles? Those would be three single phase xfers for a 3 phase service.