When you see references, especially in the ancient classics, you need to "see" the more modern term in its place. The third eye is supposed to be about the middle of the forehead just above the eyebrows. Does this provide a clue to the modern meaning of "third eye?"
The part of the brain in that location is the "frontal lobe" which involves planning/anticipation; follow-through; impulsive; judgment; reasoning; abstract thinking; smell; motor planning; personality; emotionality; speaking; integration of thought and emotion; self-monitoring. Hmm, if we look at these we realize most of what we do and how we do it comes from this area so maybe, just maybe the third eye is referring to the use of the frontal lobe in meditation, etc. to achieve true seeing into the self.
I may be stretching this a bit to elucidate on the "seeing" we accomplish through the term usage of "third eye" but it makes sense. Yes, seeing or vision in a literal sense in not handled here but we are not talking about literally seeing something but lets say, "spiritually seeing" the self with a critical "eye."
To see in all directions is not just spacial but internal as well. To see ourselves as we truly are is in all probability the most difficult task/goal we have as humans. Karate-do tries to lead us toward this understanding and with the ken-po goku-i studies maybe this is the light that shines on us to allow us to see within, in all directions.