A central question in multilingualism research is how multiple languages interact. Most studies have focused on first (L1) and second language (L2) effects on a third language (L3), but a small number of studies dedicated to the opposite transfer direction have suggested stronger L3 influence on L2 than on L1 in postpuberty learners. In our study, we provide further support for stronger L3-to-L2 than L3-to-L1 influence and show that it extends to (a) phonetics and the lexicon and (b) childhood learners. Fifty Spanish-Basque-English trilingual adults who had acquired Spanish from birth and Basque between 2 to 4 years of age through immersion participated in a speeded trilingual switching task measuring production of voice onset time and lexical intrusions. Participants experienced more phonetic and lexical crosslinguistic influence from L3 English during L2-Basque production than during L1-Spanish production. These findings show that even highly proficient early bilinguals experience differential influence from a classroom-taught L3 to L1 and to L2.