If your switch is stock the 3 positions are 'lock', 'off' and 'ignition'. In 'off' you can remove the key and still move the switch to 'ign'. I don't know what years those were used but , to ID one, it has a little tab on the face of the switch. By '53, this tab was gone and the switch just had 'locked off' and 'ign'.
Without looking at the manual I would suspect that one terminal is voltage from the battery and the other two are hot when the switch is on. One of these would go to the amp gauge and one to the coil. I'd hook the heater switch to the one going to the ammeter.
I highly doubt if the heater is in the wiring diagram in the manual.
