This is a reference to the 7 Planes of Existence, perhaps as defined by the Michael Teachings, a new aged spiritual doctrine developed around a channeled entity (or group of spirits) known as Michael.

While there actually exist several cosmologies with slightly differing views on the 7 planes of existence, nearly all of them include a physical, astral, and mental plane, with each plane believed to be higher than the one before it. KRS-One’s reference to “the final three” indicates that there are still three more planes of consciousness higher than the mental plane.
When applied here, this progression of planes suggests that KRS-One wants his listeners to look beyond the surface of his rhymes and understand his message on the highest level of consciousness.