This study explored motivation of Thai students who simultaneously studied English and additional L3s (Chinese, Japanese and Korean) in language specialist majors and in English-medium business majors in a Thai university, using Dörnyei’s (2005, 2009) L2 motivational self system and Ushioda’s (2009) person-in-context relational system. Foreign Language Anxiety (FLA) and International Posture (IP) are also investigated in order to find the dynamic interplay between these closely linked variables. The study consists of three parts: 1) a quantitative cross-sectional study with students from year 1to year 4 (N = 356), 2) retrospective interview of 14 students who were studying different languages, and 3) longitudinal case studies of five students over three years. It sheds light on the topic of motivations of multiple language learners across languages and in different learning environments. The findings show that while the increasing importance of English threatened learners’ motivation to study other foreign languages as found in previous research (e.g., Dörnyei et al., 2002, 2006; Henry 2010, 2012), the predominance of English also encourages students to study other languages. International posture was found to link to motivation to learn foreign languages other than English, while FLA does not correlate to IP at all. For both language specialist and English-medium business students, their motivation was mainly generated by immediate need and future use of that particular language.