Research on the effects of bilingualism on third language (L3) development set in bilingual communities is scarce, outdated, and has provided mixed results. This study investigated the effects of exposure and age of onset of acquisition (AOA) of L3-English, as well as first and second language (L1/L2) skills and use, in the development of L3-English lexical and grammatical receptive abilities in Catalan-Spanish bilingual learners in Catalonia. The study followed a longitudinal design with three data collection times: Times 1 and 2 took place at the onset and the end of Grade 1, respectively, and Time 3 happened at the end of Grade 2. Results showed an overall growth of vocabulary and grammar over the first two years of primary schooling. In addition to testing time, L3-English exposure outside of school predicted higher receptive vocabulary and grammar skills, whereas an older English AOA predicted lower skills. The L1/L2 variables showed different associations to the vocabulary and grammar scores. However, the minoritized language, Catalan, bore a stronger association with the two L3 outcomes than the majority language, Spanish.