[morzh]

Ok

The three wires are likely to be:

1. The end of 64 field coil that goes to cap 66 and rectifier filament and power trans pin 6.
2. The second end of 64 FC that goes to one end of the audio xfmr primary (connects to 42 Screen Grid)
3. The other end of audio xfmr primary (goes to plate of 42 tube).

Now.

Mesure ohms between any two prongs.

Write down the resistance. It can be:

1. 1.1 kOhm
2. 400 Ohm
3. 1.5 kOhm (sum of the above).

Now. Choose the combination that gives you 1.5kOhm.
Mark the pins A and B.

Measure from any of the two to the third pin which you mark as C.
Lets say If it is 400 Ohm it is your auio primary and then

pin A is your 42 plate
pin B is your FC rectifier end
Pin C is your FC connected to audio xfmr primary.

Connect
A to 42 Plate,
B to The end of 64 field coil that goes to cap 66 and rectifier filament and power trans pin 6, and
C to the Screen Grid of 42.