I'm not sure how you got 625 Amps.
The secondary of a 75kVA 3 phase transformer is 208 Amps at 208Y/120 Volts (kind of a weird coincidence, huh?).
There are 2 possible ways to connect the 3 transformer windings in a 3-phase transformer; one has each winding connected to the next and forms a triangle when drawn (this is the Delta connection) and the other has 1 end of each winding connected together and forms a 3-pointed star when drawn (this is the Wye connection).
The square root of 3 comes about because the ratio of the magnitude of the voltage vector between phases and the magnitude of the voltage vector between any phase and the common point (Neutral) is equal to the square root of 3.
Take a look at this pdf from Jefferson transformer and see if this helps...sometimes the vectorial math involved in electricity gets confusing.
http://www.jeffersonelectric.com/s3/site/catalog/basics.pdf
