Viral Infection: At 3 months old, viral infections are common due to an immature immune system and exposure to various pathogens. Symptoms can range from mild to severe and often include fever, which is a typical response to viral infections in infants.

Urinary Tract Infection (UTI): UTIs are a significant concern in infants, especially those under 3 months, as they can lead to severe complications if not promptly treated. Fever can be the only presenting symptom in non-toilet-trained children.
Otitis Media: Ear infections are common in infants and can cause fever, irritability, and pulling at the ears.
Gastroenteritis: Viral gastroenteritis can cause fever, vomiting, and diarrhea in infants.

Bacterial Meningitis: Although less common, bacterial meningitis is a medical emergency that requires immediate attention. Fever, irritability, and lethargy can be presenting symptoms, and it's crucial to consider this diagnosis in any febrile infant under 3 months.
Sepsis: Sepsis is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. Fever, along with other signs of infection, can be an indicator.
Pneumonia: Bacterial or viral pneumonia can present with fever, cough, and difficulty breathing in infants.

Kawasaki Disease: An acute febrile illness of childhood characterized by inflammation in the walls of medium-sized arteries throughout the body. It's rare but important to consider due to potential cardiac complications.
Congenital Infections: Such as congenital syphilis, toxoplasmosis, rubella, cytomegalovirus (CMV), and herpes simplex virus (HSV), which can present with fever among other symptoms in newborns and infants.
Inborn Errors of Metabolism: Certain metabolic disorders can present with episodes of fever, among other symptoms, due to the accumulation of toxic substances.