藏族学生英语三语（L3）习得受到母语藏语（L1）和第二语言汉语（L2）的双重影响。以多语交互影响研究者提出的类型近似模型、第二语言主导模型和累积增强模型为理论基础，收集L2水平不同的藏族L3学习者的作文语料，从流利度、词汇丰富度和词语错误率等三个维度对比分析。结果表明L1和L2在L3写作中发挥了不同的作用：L1在作文构思阶段发挥作用；L2水平越高，词汇错误率越低，但L2水平在词汇丰富度上并不具有区分力；教学媒介语对L3也会产生一定影响。这一结果对于藏区英语教学具有一定的启示。

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.