A third heart sound is, like a raised jugular venous pressure, on of the important signs of cardiac failure. You can get third heart sounds in both the left and the right ventricle. The cause was a mystery for quite some time, but consensus has gradually settled on the fact that it represents the sudden deceleration of blood as the ventricle fills. Conditions that "overfill" the heart, or that fill it too quickly, produce a third heart sound.
I'm afraid that I'd best quote at length from "Pathophysiology of Disease" (4th edn.) by McPhee, Lingappa and Ganong:
The third heart sound is a low-pitched sound that is heard during rapid filling of the ventricle in early diastole. The exact mechanism responsible for the genesis of the third heart sound is not known, but the sound appears to result either from the sudden deceleration of blood as the elastic limits of the ventricular chamber are reached or from the actual impact of the ventricular wall against the chest wall. Although a third heart sound is normal in children and young adults, it is rarely heard in healthy adults over 40 years of age. In these individuals, the presence of a third heart sound is almost pathognomonic of ventricular failure. The increased end-systolic volumes and pressures characteristic of the failing heart are probably responsible for the prominent third heart sound.
Harrison's (16th edn.) says much the same thing, stating that the sound is produced "at the termination of ventricular filling", and usually indicates "impairment of ventricular function, AV valve regurgitation, or other conditions that increase the rate or volume of ventricular filling."
Hope that makes sense!