The study addresses the degree of transfer of literacy dimensions of academic proficiency (AP), that is, reading comprehension and writing, across three languages--Russian (L1), Hebrew (L2), and English (L3)--and investigates whether a common conceptual source underlies the linguistic and cognitive operations of the trilingual learner (Grosjean, 1985). The sample included 274 Russian-speaking 11th graders studying Hebrew (L2) and English (L3). The instruments for assessing AP were (a) an L1 writing task and "can do" questionnaire and (b) AP tests in L2 and L3. Questionnaires were administered to gather information about the participants' background variables. The results indicate interdependence of AP across the 3 languages. AP in L1 contributed significantly to the prediction of AP performance in L2 and L3 independent of interaction with arrival age, gender, and socioeconomic status. Reading comprehension and writing in Hebrew (L2) significantly predicted performance on the parallel measures in English (L3). L3 writing proved to be a better predictor of AP in L2 compared to L3 reading comprehension and predicted not only L2 writing but also L2 reading. The results indicate that the Interdependence Hypothesis transpires in the context of 3 languages and can be interpreted to comply with holistic views of bi/trilingualism (Cummins, 2008; Jessner, 2008c).