Back in the 1950's some old Yoruba stories were published by the writer Amos Tutuola. These were old Yourba stories (in what is today Nigeria), and may have a direct link to honest-to-the-gods Voudoun. He spoke of a man having three minds... the first being characterized as brash and quick thinking but frequently wrong (but good for morale and honor), the second being thoughtful and analytical, and frequently right (but slower); the third being almost a being unto itself. It doesn't answer often, but when it does, you listen. It is described as almost a demigod like being who looks out for you, preforms great feats of strength when all of your senses tell you that you have no strength left. People often left offerings to it, and the like. I believe it was called "the Supreme Second." Now in the stories I've read, it doesn't explain much about it, so it could be that I've totally misunderstood the concept. One gets the sense of a guardian angel or spirit guide, but it definitely dwells within the individual.
I suspect that the Third Mind is in fact what the New Agers call "The Higher Self" and what some Hermetics call "The Holy Guardian Angel" and what Crowley called "The Aeon".
Another interesting reference of a similar concept comes from Buddhism. The idea of the Wise Mind, balanced with the Analytical Mind and the Emotional Mind. This may also be echoed in some of Jung's writings.
The idea is that the Wise mind is a balance between the Emotion Mind and the Analytical Mind, and an arbiter of Inner Wisdom and enlightenment.
For what it's worth, I have found this concept to be very valuable when judging those thoughts and inner voices when delving deep in the psyche, or even just figuring out what to do in every day life. You can also find these concepts in a program called "Dialectical Behavioral Therapy" or DBT, which is a program for thinking effectively and problem solving for people with Mood Disorders and the like. Probably the only piece of non-bullshit to come from the psychiatric establishment in a LONG time. It's the only therapy program with real results that I've ever seen.