Third heart sound or S3 is a low-frequency heart vibration which occurs in the early diastole after filling of either ventricle. It is normal physiological occurrence in individuals below 40 years. However, in individuals above 40 years it is an abnormal heart sound.Third heart sound is caused by increase in atrial pressure which indicates increased blood flow rates. Third heart sound is also known as ventricular gallop, protodiastolic gallop and ventricular filling sound. Third heart sound is caused by - Congestive Heart Failure (CHF) - Cardiomyopathy - Valvular regurgitation - Left-to-right shunts