Viral Gastroenteritis: This is the most common cause of diarrhea, vomiting, and stomach pain, often due to norovirus or rotavirus, especially in children, but also common in adults. The symptoms usually resolve on their own within a few days.

Food Poisoning: Bacterial infections from contaminated food can cause these symptoms. Common culprits include Salmonella, E. coli, and Campylobacter.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with chronic diarrhea, abdominal pain, and sometimes vomiting.
Irritable Bowel Syndrome (IBS): Although IBS doesn't typically cause vomiting, it can cause chronic abdominal pain and changes in bowel habits, including diarrhea.

Appendicitis: While the classic presentation is abdominal pain that migrates to the lower right quadrant, some patients may experience nausea, vomiting, and diarrhea. Missing this diagnosis can lead to perforation and severe complications.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia. It's more common in children but can occur in adults. Symptoms include severe abdominal pain, vomiting, and sometimes diarrhea.
Mesenteric Ischemia: A reduction in blood flow to the intestines can cause severe abdominal pain, vomiting, and diarrhea. This condition requires immediate medical attention to prevent tissue death.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, abdominal pain, and weight loss, among other symptoms.
Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to chronic diarrhea, abdominal cramps, and weight loss.
Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine, causing chronic diarrhea, abdominal pain, and malabsorption.

Each of these diagnoses has distinct features and requires a thorough medical evaluation for accurate diagnosis and appropriate treatment.