Using data from the Fragile Families and Child Wellbeing Study (N = 4,898), this study investigated how the share, correlates, transition patterns, and duration of 3-generation households vary by mother's relationship status at birth. Nine percent of married mothers, 17% of cohabiting mothers, and 45% of single mothers lived in a 3-generation family household at the time of the child's birth. Incidence over time was much higher and most common among single-mother households: Sixty percent lived in a 3-generation family household at least 1 wave. Economic need, culture, and generational needs were associated with living in a 3-generation household; correlates varied by mother's relationship status. Three-generation family households were short lived, and transitions were frequent. Kin support through coresidence was an important source of support for families with young children and in particular families in which the parents were unwed at the time of their child's birth. (Contains 4 tables and 1 figure.)