1. What is diarrhea in three-month-old babies? Three-month-old baby diarrhea refers to diarrhea symptoms in infants and young children within 3 months. This condition is relatively common and is usually caused by incomplete digestion or infection. What are the symptoms of diarrhea in 2 and 3 month old babies? 1. Diarrhea: The number of bowel movements per day increases,