Final answer:
In assessing respirations in a 3-month-old, the nurse should observe the abdominal muscles as they are the primary breathing muscles in infants, due to the diaphragmatic breathing pattern common in this age group.
Step-by-step explanation:
When assessing respirations in a 3-month-old, the nurse would assess respirations by observing the abdominal muscles. This assessment is done because in infants and young children, abdominal muscles play a primary role in breathing rather than the thoracic muscles, which are more important in older children and adults. This is partly because infants have a diaphragmatic breathing pattern, and the movement of the diaphragm is more noticeable in the abdomen. The intercostal muscles assist in breathing by changing the dimensions of the rib cage, but they are not the primary muscle group assessed in a 3-month-old for respirations.