Shared intentionality theory posits that at age 3, children expand their conception of plural agency to include 3- or more-person groups. We sought to determine whether this conceptual shift is detectable in children's pronoun use. We report the results of a series of Bayesian hierarchical generative models fitted to 479 English-speaking children's first-person plural, first-person singular, second-person, third-person plural, and third-person singular pronouns. As a proportion of pronouns, children used more first-person plural pronouns, only, after 3;0 compared to before. Additionally, children used more 1pp. pronouns when their mothers used more 1pp. pronouns. As a proportion of total utterances, all pronoun classes were used more often as children aged. These findings suggest that a shift in children's social conceptualizations at age 3 is reflected in their use of 1pp. pronouns.