This empirical study aims to shed light on L3 initial-stage transfer and later development by investigating Q-operations in L1 English–L2 Cantonese and L1 Cantonese–L2 English bilinguals’ L3 Mandarin and L1 English speakers’ L2 Mandarin at low and high proficiency levels. Data from an online cross-modal priming task and an offline acceptability judgement task found that structural similarity determines transfer source selection. Adopting a de-compositional approach to cues of different domains, we have found both facilitative and detrimental transfer effects from Cantonese, with the latter triggered by orthographic and phonological cues. Our data also suggest that detrimental transfer effects can persist at an advanced stage and that L3 development and acquisition results can be affected by various factors such as word frequency and the nature of learning situations.
