A 6-year-old boy is admitted to the hospital with coughing and dyspnea. During taking of the history he complains that it feels like there is glass in his lungs. Auscultation reveals abnormal lung sounds. The abnormal lung sounds are heard most clearly during inhalation with the scapulae abducted. Which of the following form the borders of a triangular space where one should place the stethoscope in order to best hear the lung sounds?