A Neutral Grounding Transformer is NOT a three phase transformer, but a single phase transformer, with the primary (HV) rated voltage equal to the system phase-to-neutral voltage and the secondary (LV) rated voltage either 110V or 240V.
Why is it required? For economic reasons. Let us see how. Supposing you have an 11kV System, whose neutral you want to ground through a resistance. The desired ground fault current, let us say, is 10A. Now, if you want to connect a resistor directly in the path between the system neutral and earth, the value of the resistor would be (6350/10 = 635 Ohms) and the voltage rating of the resistor would be 6350V. A 6350V, 635 Ohms resistor would not be cheap.
Now, if you opt for a single phase 6350V/240V, neutral grounding transformer, whose 6350V winding is connected in the neutral to ground path, you can connect a simple 0.9 Ohms resistor across the 240V secondary of this neutral grounding transformer. This 0.9 Ohms resistor at 240V side will reflect multiplied by the square of the turns ratio, at the HV side (i.e.) 6350/240 is 26.45 whose square is 700. The 0.9 Ohms resistor connected across the 240V secondary of the neutral grounding transformer would appear as (700 x 0.9 = about 630 Ohms). And, the added advantage is that this resistor needs to be insulated only for 240V. A reduced Ohmic value resistor, with a reduced insulation rating is cheaper. And, the neutral grounding transformer can be short-time rated, to optimize on the size & cost of the neutral grounding transformer.