There are a number of potential causes of your chronic diarrhea
. One helpful step in narrowing down the cause is determining if the problem is due to over-secretion of a substance within the body, or if there is some external cause bringing about the problem. In the first case, fasting for a day typically does not make any difference in your stool output, and the diarrhea often occurs at night or wakes you up from sleep. In the latter case, you may notice that reducing your dietary intake stops your diarrhea and allows you greater quality of life.
If the first case is true, then your body may be secreting some endogenous substance at a higher rate than normal. These causes are very rare, and include syndromes of excess chemicals such as VIP, serotonin and 5-HIAA. Excess bile acid secretion from the liver can also present this way (often when the gallbladder has been removed). Long-standing infections would also be unusual given the time course, but could be possible and stool studies can help diagnose this. Lastly, medication side effects are often overlooked, and are important to consider.
In the second case, food-related diarrhea is commonly diagnosed. This is the case with lactose intolerance and celiac disease.
If you have truly been having diarrhea for three years and have yet to find an answer, it would be advisable that you consult with a gastroenterologist
for further work-up, which should ultimately include either a flexible sigmoidoscopy or colonoscopy