It is the connector in the middle. It should be a 4-point connector. Points are labeled 15, 50, LA, and 31 on the relay box. Here is the mating diagram:

Point 15 <-> Jack #1 (to fuse box terminal 15)
point 50 <-> Jack #2 (to starer lockout and backup lamp switch terminal 50)
point La <-> Jack #3 (to Indicator Lamp)
point 31 <-> Jack #4 (to ground)

The above info is from the service manual and is good for 617 engine starting 1981.

I do not know why you have only three wires. Are you sure about that?

Keep in mind that the plugs are wired together inside the relay box. If you did not unplug the relay box, you was just measuring the same circuit five times.

David