3-Phase Transformer Model

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.

How are the transformers connected? Wye or delta?

Connected delta-wye.

Edited: Sorry I misremembered. Connections make no difference. 1.9% using 100 kVA and line to neutral voltage to get ohms secondary equates to 1.9% using 300 kVA and line to line voltage.

No matter how the 3 single phase transformer connected, the impedance should be the same as the single phase tested value

What is one of the transformers has a different nameplate impedance?

Then there is no equivalent three phase impedance.

Can 3 single phase transformers with different IZ% be connected to a transformer bank?

It is my understanding that they can be, but they still have to be within a very tight range..