Hi again Barbara:
A 3rd heart sound is caused by rapid filling of the ventricle during diastole (relaxation phase). Its presence could be due to many things (some normal, some not so normal), so depends on what other symptoms you have. Is this a new finding made by your doctor? A note about S3 sounds - their detection is dependent on the skill level of the clinician who noted them. I would question its presence if the person who "heard" it was anyone but a competent cardiologist. It is often confused with a split S1, which would indicate something entirely different. I guess what I'm saying is it would be fruitless to speculate on what might cause a certain heart sound, without knowing for sure whether it was really there. Sorry I can't be of more help.