S3 is a dull, low-pitched sound best heard with the bell placed over the cardiac apex with the patient lying in the left lateral decubitus position.

This heart sound when present in a child or young adult implies the presence of a supple ventricle that can undergo rapid filling.

Conversely, when heard in a middle-aged or older adult, an S3 is often a sign of disease, indicating increased ventricular filling due to congestive heart failure or severe mitral or tricuspid regurgitation.

[citation needed] Recent research suggests that mitral valve annulus diameter is one of the more important factors in producing the S3 sound.

[citation needed] In conditions affecting the pericardium or diseases that primarily affect the heart muscle (restrictive cardiomyopathies) a similar sound can be heard, but is usually more high-pitched and is called a 'pericardial knock'.The S3 can also be confused with a widely split S2, or a mitral opening snap, but these sounds are typically of much higher pitch and occur closer to the onset of S2.