What kind of power supply is this, AC/DC? Is this on the DC side of the transformer? Generally there's only two cables on the DC side, not three - is the connector proprietary?
I doubt it's on the AC side since standard AC coloring is white and black. It's either going to be a ground or a positive lead (obviously). Since you've got the adapter torn up anyway, why not just plug it to a multimeter and find out?