Tibetan students’ learning of English (L3) receives a multilingual influence from both their moth tongue (L1) and Chinese (L2). Implicated by the three hypothesized models depicting the roles of L1 and L2 in L3 acquisition, the L2 Status Factor Model, the Cumulative Enhancement Model, and the Typological Proximity Model, this study attempts to find out what roles L1 and L2 play in Tibetan students’ learning of L3. A contrastive analysis of writing texts by the students of higher L2 proficiency and by the students of lower L2 proficiency revealed：1) L1 played a significant role in formulating a writing task; 2) that while higher L2 proficiency seems to be able to reduce lexical errors, it cannot distinguish writings of different degrees of lexical variety; 3) and that L3 teaching medium language seems to be involved into the multilingual process too. The results may have some practical implications for L3 teaching in Chinese Tibetan areas.