Diarrhea, stomach pain, and nausea for three days now. what should i do?

Gastroenteritis: Seems like you have gastroenteritis which may be viral or bacterial, Try probiotics. Hydrate, drink 3-4 liters/day gatorade/water. White rice. toast avoid dairy . If not to be improving (24 hours) f/u with doctor /urgent care for stool cultures for camphylobacter. shigella, salmonella, E coli, H pylori and C diff ( if recent antibiotics). No alcohol, check for hepatitis