The ignition warning light earths through the alternator windings.
In doing so the windings form a magnetic field.
When the alternator turns this magnetic field aids the production of electricity.
So we call the current( from the ignition light) "excitation current".
If you spin the alternator fast enough it may "self excite".
To connect you ignition warning light you need to join two of the wire at the 3aw.
Not certain without digging out the wiring circuit, but I think the third wire is an earth.
I would expect between 12.2 and 12.8 volts at the battery with everything switched off, and between 13 and 14 volts with the engine running.
Hope this helps.
Jim.