The purpose of this study was to investigate the factors predicting academic proficiency (AP), the specialised domains required for performing academic tasks, among Russian speaking (L1) immigrants currently studying Hebrew as a second language (L2) and English as a third language (L3) in Israeli schools. Specifically, the study examined the contribution of "demographic," "linguistic," and "social-psychological variables," as well as the "AP level in first, second and third language" to the prediction of AP in L2 and L3. The sample included 274 eleventh grade students. The instruments for assessing AP included a writing task in L1, and AP tests in L2 and L3. Questionnaires were administered to collect information regarding the participants' demographic, linguistic and social-psychological variables. The results indicated that gender, arrival age, current language use, perceived parental aspirations, L3 writing ability and studying L1 at school significantly predicted AP in L2, while only age at onset, current language use and L2 writing ability predicted L3. Writing ability in L1 was a significant predictor of reading and writing abilities in both L2 and L3, even after controlling for all background variables. Studying L1 at school contributed to the prediction of L2 reading ability only, holding all other variables constant.