Drawing on qualitative research, this study explores (1) how Hong Kong university students' existing L1 and L2 affect their learning motivation for an L3 and (2) how an individual's L2 motivation changes because of learning an L3, from a dynamic systems theory perspective. Eight undergraduate students from Hong Kong participated in this study. They were all native Cantonese speakers with English as their second language (L2) and Japanese as their third language (L3) at different proficiency levels. The students attended two rounds of extensive semi-structured interviews that focused on their sources and changes of motivation, their own perceptions of their L2 and L3 selves, their individualised methods and experiences in learning the two languages and their understanding of their cultural identities. The findings reveal the complexity of multilingual learners' motivational dynamics in learning L1, L2 and L3. Furthermore, the influence of an L3 on the existing L2 motivation is shown to be an intriguing but under-investigated research area. Our results lend support to the newly proposed dual-motivation system theory and should contribute to a more nuanced understanding of multilingual learning.