If a 6-month-old baby has not had a bowel movement for 3 days and their stomach is not hard, it may not be a cause for concern. However, it is important to monitor the situation and make sure the baby stays hydrated. Encouraging the baby to drink more fluids and consume foods high in fiber, such as sweet potatoes, fruits, vegetables, and oatmeal, can help soften their stools. Additionally, gentle exercises and massaging the baby's stomach in a clockwise direction may also help alleviate constipation. If the issue persists or if there are any other concerns, it is best to consult a healthcare professional for further advice.