Despite the increasing prevalence of 3-generation family households (grandparent, parent, child), relatively little research has studied these households during early childhood. Using nationally representative data from the Early Childhood Longitudinal Study--Birth Cohort ("N" = ~6,550), this study investigated the associations between 3-generation coresidence in early childhood and school readiness, and how the associations differed by maternal age, race/ethnicity, nativity, relationship status, and poverty. For the full sample of children, no associations between 3-generation coresidence and school readiness were found. Analyses by demographic characteristics found that race/ethnicity and nativity moderated the associations, whereas maternal age, relationship status, and poverty did not. The study found that 3-generation coresidence was associated with lower levels of expressive language for White, Asian, and Black children but more expressive language for Hispanic children. Coresidence was also associated with more externalizing behavior for White and American Indian/Alaskan Native children but less externalizing behavior for Hispanic and Black children. Analyses by maternal nativity found that for children of immigrant mothers, 3-generation coresidence was associated with more expressive language and less externalizing and internalizing behaviors. Interactions between race/ethnicity and nativity found that the positive associations for Hispanic children were concentrated among children of immigrant parents. No differences were found between grandmother-only and grandmother/grandfather 3-generation family households. Overall, the findings suggest there may be heterogeneity by race/ethnicity and nativity in the associations between 3-generation coresidence and school readiness.