This article reports a latent variable study exploring the specific links among executive processes of working memory, phonological short-term memory, phonological awareness, and proficiency in first (L1), second (L2), and third (L3) languages in 8- to 9-year-olds experiencing multilingual education. Children completed multiple L1-measures of complex span, verbal short-term storage, and phonological awareness, and tests of proficiency in a range of linguistic domains (vocabulary, grammar, and literacy) in Luxembourgish (L1), German (familiar L2), and French (unfamiliar L3). Results indicate that executive processing abilities, phonological short-term memory, and phonological awareness operate as distinct but related constructs that manifest differential associations with native and second language proficiency in multilingual children: Phonological short-term memory was uniquely linked to vocabulary in L1 and the structurally similar L2; executive processes were related to grammar across languages, reading comprehension, and spelling; and phonological awareness made specific contributions to word decoding, spelling, and language proficiency in the structurally dissimilar L3. Phonological processing abilities appear to be critical for acquiring the sound structure of a new language, whereas executive processes share more general links with higher order linguistic abilities in second language learners. (Contains 3 tables, 1 figure and 1 footnote.)