Option 1: During inhalation, diaphragm relaxes. This statement is incorrect. During inhalation, the diaphragm contracts and moves downward, creating a vacuum in the chest cavity to allow air to enter the lungs.

Option 2: During exhalation, thorax cavity expands. This statement is incorrect. During exhalation, the thoracic cavity actually decreases in volume as the diaphragm relaxes and moves upward, pushing air out of the lungs.

Option 3: During inhalation, intra-pleural pressure becomes more negative. This statement is correct. During inhalation, the diaphragm contracts, creating negative pressure in the pleural cavity. This negative pressure helps in expanding the lungs and drawing air into them.

Option 4: Unlike inhalation, normal exhalation is an active process. This statement is incorrect. Both inhalation and exhalation involve active muscle contractions. During normal exhalation, the diaphragm relaxes and the intercostal muscles between the ribcage contract, pushing air out of the lungs.

Therefore, the correct answer is option 3, as it accurately describes the changes that occur during inhalation.