To examine the development of triplets, 23 sets of triplets were matched with 23 sets of twins and 23 singletons (N=138). Maternal sensitivity was observed at newborn, 3, 6, and 12 months, and infants' cognitive and symbolic skills at 1 year. Triplets received lower maternal sensitivity across infancy and exhibited poorer cognitive competencies compared with singletons and twins. The most medically compromised triplet showed the lowest regulation, received lower maternal sensitivity, and demonstrated the weakest outcomes compared with siblings. Structural modeling charted three levels of influence on cognitive outcomes: direct, indirect, and contextual. The triplet ecology provides a context for assessing the relations among infant inborn dispositions, the rearing environment, and the role of exclusive parenting in development.