Multilingual perspectives have become the mainstream research paradigm in language motivation studies lately. The motivational interplay between English and French, as two important foreign languages in China, has received little academic attention, but this topic is becoming increasingly significant for language education in the era of global English. This study set out to explore the motivational differences between English majors and non-English majors learning French as the second foreign language (SFL) in a Chinese university, and identify the influencing factors of learners' motivation. A motivation scale was designed in the form of questionnaire, and participants (n = 56) from two classes were tested. Variation sampling was then adopted for interviews to capture learners' multiple motivational profiles. The results reveal that the learners' SFL motivation fluctuates constantly with their first foreign language being English. Overall, the non-English majors have a significantly higher motivation than English majors. This proves that the conclusion in the literature of L2 English having a negative impact on L3 motivation is oversimplified. We found it applies to English majors, but not to non-English majors. We also proposed an original three-dimensional motivation model, taking into account the influencing factors at the macro, meso and micro levels. It helps to reveal the dynamic interplay of different motivational forces. Teachers are suggested to help maintain and promote learners' motivation by constantly strengthening supporter language awareness and emphasizing the benefits of being multilinguals.