This paper investigates to what extent stable and unstable three-generation family households (grandparent, parent and child) are associated with child socioemotional, cognitive and health outcomes over the first three years of a child's life. Using longitudinal data from the Fragile Families and Child Wellbeing Study (N=2,666) differences in the association by mother's relationship status and interactions with nuclear family instability are investigated. Results suggest stable three-generation family households are associated with child wellbeing whereas unstable or transitory three-generation households are not. Living in a stable threegeneration family household is associated with more externalizing and internalizing behavior problems as well as higher odds of being overweight. Stable three-generation coresidence is also associated with higher verbal scores. Differences by mother's relationship status reveal a protective association with externalizing behaviors for stably partnered mothers but few interactive effects between nuclear and three-generation family instability. Overall, the results suggest that stable, but not unstable, three-generation family household coresidence is associated with child wellbeing.