The third heart sound is caused by early diastolic filling due to ventricular relaxation, shortly after closure of the aortic valve (corresponds to Y descent in JVP). It may be normal in children and young /middle aged adults.

Causes of an abnormal third heart sound are: Left ventricular failure

Severe MR and TR VSD, PDA

Constrictive pericarditis Hypertrophic cardiomyopathy Dilated cardiomyopathy

AV fistula thyrotoxicosis