According to Osho, in Buddha's map of inner bodies the mental body is the second sheath: the field of thoughts, beliefs, and conditioning. Enlightenment begins by transcending this layer—ceasing identification with mind—so awareness can move beyond it into the third, astral body of pure light. Therefore, the 'third body' is not the mental body.
It's the second layer of you—your thoughts and beliefs—that you must watch and go beyond to awaken.