Drawing on the notions of ‘interface’ and ‘cross-linguistic influence’ in second language acquisition (L2A), the present study addresses the possible role of French as a third language (L3) in the L2A of English number agreement with two major concerns: (1) the degree to which third language acquisition (L3A) will bring about any positive or negative influence on L2A, and (2) the way in which an L3 interacts with L2 and/or even L1 on narrow syntax or an internal interface as identified in L2A.

To address the research concerns, a comparison was made between 48 L1 Cantonese–L2 English–L3 French (CEF) participants and 46 L1 Cantonese–L2 English (CE) participants. Twenty English native controls were also recruited. All participants completed two timed tasks: a grammaticality judgement-correction task and a free writing task. After the experiment proper, the grammatical component of the Oxford placement test, consisting of 60 items, was administered to the two groups of L1 Cantonese speakers.

The following tests were run: the Wilcoxon signed ranks test, the Mann–Whitney U test, the Kruskal–Wallis one-way analysis of variance, the non-parametric x2-test and the two-sample Kolmogorov–Smirnov test.

While the placement test indicated comparable English levels between the two L1 Cantonese groups, the free writing task revealed certain distinctive patterns, suggesting possible influence from L3 French to L2 English among the CEF participants. In particular, the advanced L3 French participants were different from their L3 French peers and the CE participants in their free production of grammatical nominal plurals.

The study highlights the potential of reverse cross-linguistic influence from L3 to L2 among L1 Cantonese speakers.

Possible traces of L3 French–L2 English effect were noted in affecting the CEF learners’ use of English plural morphology by ‘neutralising’ their production of missing or redundant plural ‘-s’. Typological similarities between French and English and L3 proficiency were found to play a role in such transfers.