You have asked about this powerful passage from the Sermon on the Mount: "The eye is the lamp of the body. So, if your eye is sound, your whole body will be full of light; but if your eye is not sound, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!"
Your specific question is whether this might be a reference to a spiritual “third eye” located between the brows. I think that's a perfectly reasonable assumption. Jesus is clearly not speaking of our two physical eyes, which see a dualistic world, but of a higher spiritual sight that sees, knows and affirms the Light of God expressing in and through all things. That spiritual eye is the “lamp of the body;” it allows us to see everything as it truly is, not as it may appear to our limited human perception. If we ignore that spiritual perspective and accept only the information we receive from our human senses, then we will, indeed, be blundering around in a spiritual darkness of our own making.
The “third eye” location defined by eastern religions was, for Charles Fillmore, the center of the divine power of imagination—which I understand to be a power of creative perception, the ability to see light where our limited human sight might see only darkness, and infinite spiritual possibility where our human sight would see challenges and shadows.