The average weight of a 3-month-old baby boy is 14.1 pounds, and the average weight of a 3-month-old baby girl is 13.3 pounds. While these are the averages, a baby's weight can vary from these figures quite considerably.
Height and weight charts are available to track the development of young children. These charts show the average values and normal range of values for height and weight at each month of age up to 24 months. The charts or tables are easily available on the Internet or from child health centers. If a child weighs less than the lower bound of the normal range or more than the upper bound, seeking the advice of a pediatrician is recommended.