RJohnson wrote:

If I have a 300 kVA utility transformer bank made up of 3 single phase 100 kVA transformers each with an impedance of 1.9%, is the equivalent impedance of the 300 kVA transformer simply 3 x 1.9, or is it the square root of 3 times 1.9?

1.9%. The impedance is a per phase value.