Gastroenteritis: This is a common condition characterized by inflammation of the gastrointestinal tract, often due to viral or bacterial infections. The symptoms of epigastric pain, fever, and vomiting are classic for gastroenteritis, making it the most likely diagnosis.

Peptic Ulcer Disease: Conditions such as gastric or duodenal ulcers can cause epigastric pain and may be accompanied by vomiting. Fever can be present if there is an associated infection or perforation.
Acute Pancreatitis: Inflammation of the pancreas can cause severe epigastric pain, vomiting, and fever. The pain often radiates to the back and worsens after eating.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones, can cause right upper quadrant or epigastric pain, fever, and vomiting.

Appendicitis: Although appendicitis typically presents with right lower quadrant pain, it can occasionally cause epigastric pain, especially if the appendix is located in a retrocecal position. Fever and vomiting are common accompanying symptoms.
Perforated Viscus: A perforation in the gastrointestinal tract can lead to peritonitis, presenting with severe abdominal pain, fever, and vomiting. This is a surgical emergency.
Mesenteric Ischemia: Reduced blood flow to the intestines can cause severe abdominal pain, vomiting, and fever. It is a critical condition that requires prompt diagnosis and treatment.

Infectious Mononucleosis: Caused by Epstein-Barr virus, this condition can lead to fever, vomiting, and abdominal pain due to splenomegaly or hepatitis.
Hepatitis: Inflammation of the liver can cause epigastric or right upper quadrant pain, fever, and vomiting, especially in the acute phase.
Abdominal Migraine: A condition that causes recurrent episodes of severe abdominal pain, often accompanied by other symptoms like vomiting and fever, primarily in children and adolescents.