This study explored the nature of transfer among bilingual vs. trilinguals with varying levels of competence in English and their previous languages. The hypotheses were tested in writing tasks designed for 75 high (N= 35) vs. intermediate (N=40) proficient EFL learners with Turkish, Persian, English and Persian, English linguistic backgrounds. Qualitative data were also collected through some think aloud procedures. The findings revealed: 1) a significantly positive relationship between writing skills of the languages known by participants; 2) trilinguals performed significantly better than bilinguals in English writing tasks, although, qualitative data revealed that both groups used the same language, i.e. Persian, as the most frequently referred language and; 3) the higher-level writers tended to present less cross-linguistic influence than the lower level writers. The results are discussed in relation to different patterns of linguistic Interdependence Hypothesis and implications for language teaching in multilingual contexts.