Third Heart Sound S3:  Unlike the S1 and the S2,
the S3 is not caused by closure of any of the heart’s valves.  Instead, it is caused by vigorous passive filling of the ventricle early in diastole, i.e. shortly after the S2.
When this early filling stops, the kinetic energy from the rapidly inflowing blood is transduced into sound energy.

If present, an S3 can be used to rule-in
a diagnosis of left ventricular dysfunction.
Marcus et al. JAMA 2005.