Learner motivation is considered as one of the important factors that help to understand learner performance. As motivation is affected by both individual and social factors, it is essential to explore the learner motivation before effective instrumental programs for learners studying in different social contexts are designed and implemented. Using the expectancy-value theory (EVT) model of achievement motivation, proposed by Eccles and her collaborators, this study aimed at exploring (1) the most influential effective variables in learning English as an L3 and (2) individual factors that influence the learners' L3 motivation. The participants in this study were ethnic minority students of a university and a middle school in the Xinjiang Uyghur Autonomous Region of China. Two hundred and ninety-nine students from one of the top universities and a prestigious middle school in Ürümqi voluntarily answered a motivational questionnaire and filled in a personal information sheet. Results of the study showed that L3 learners held higher utility and attainment value than intrinsic value and expectancy for success. The age onset of bilingualism, L2 exposure and attitude towards L3 learning are the important individual factors related to learners' L3 motivation. On the basis of these results, the author provides suggestions on the provision of L3 education, the medium of L3 instruction, as well as the influence of present bilingual education policy on teaching English as an L3 in the context of different bilingual teaching modes in the Xinjiang Uyghur Autonomous Region.