transformer 3 taps in the primary
Assuming that this is a step down transformer with turns ratio (N:1);
Try randomly applying low voltage (e.g.; 10mV at appropriate frequency) AC and measure signals at other pins by using an oscilloscope. When you see N times amplified version of your input signal (i.e.; 10NmV), the pins you see the amplified AC are the primary windings. Now, apply a higher test voltage (e.g.; 10V) to the primary windings. The other pins are from secondary winding; observe the voltage levels on them to find out the tapping scheme.