As usual Roy nailed it.
The way I'd do it is to disconnect the 3 yellow wires then test across the 3 tabs that the yellow wires plug into, I've just always did it with alternator open circuit, I've never tried in circuit, but it should also give you a high AC voltage, with that voltage rising as revs increase.
Probably something around 35-40V AC at 3k and they should all be identical.
John