The third heart sound is heard early in diastole. Along with the first and second heart sounds, this extra sound creates a gallop cadence and sounds like "Kentucky."
In this auscultation example the second heart sound is unsplit to make it easier for you to distinguish the third heart sound.
The third heart sound is very low frequency (between 25 hz and 50 hz). Listen with the bell of the stethoscope at the cardiac apex. Asking the patient to lie on his left side will frequently increase the intensity of the third heart sound.
A third heart sound is heard on occasion in normal individuals with no cardiac pathology.