What are the voltages that are supposed to be coming out of these three wires?
First- The 3 yellow wires MUST BE disconnected from the wires leading to the voltage regulator to get a valid voltage measurement..
That 78 GoldWing has a lower output stator (charging system) that the later 1200 bikes so the A/C voltage from the 3 yellow wires might be a little lower than the typical 50 volts+ per leg but should probably at least be over 40 volts A/C & about the same between all 3 legs (yellow wires) at 4000 engine RPM's.
Disconnect the 3 yellow wires at the connector (or cut them) at the front of of the battery area,, then label the 3 wires coming from the rear of the engine A,B,C,, then use a voltmeter set on the 100 volt A/C (alternating current) scale & while running the engine at 4000 RPM's or higher measure between A to B, B to C, &C to A.. Probably need to see 40 A/C volts or higher & about the same from all.
You can also measure each (individually) for resistance to ground. You need close to infinity there (open), then measure the resistance between each wire to the others (probably need to see 1-3 ohms there (or at least close to the same anyhow).