The study considers (1) the nature of multilingual transfer in the pre-intermediate stage of third-language (L3) English and (2) the upper limit of L3 ultimate attainment with respect to the acquisition of definite and indefinite restrictive relative clauses.

The methodology used was four-point scale acceptability judgment tasks testing (un)-grammatical relative sentences.

The accuracy scores of two control groups of French (n = 15) and English (n = 12) natives and two groups of pre-intermediate (n = 11) and advanced (n = 15) adult learners of L3 English are submitted to parametric statistical analysis.

The results of the pre-intermediate L3 learners indicate that the relative complementizer phrase structure is available as a block from the earlier stages, while the feature matrix of the complementizer is a hybrid of first-language (L1) Arabic and second-language (L2) French features [EPP, ±definite, –wh]. The L3 interlanguage at this stage presents simultaneous L1 non-facilitative and L2 facilitative transfer effects. The performance of the advanced L3 learners shows that they can successfully re-assemble the features of the complementizer matrix substituting the target [–wh] for the native [±definite].

This article uses a novel language triplet L1 Arabic–L2 French–L3 English to investigate the L3 acquisition of restrictive relatives in a formal foreign language context. The focus is on the interplay between the (in)-definiteness of the head noun and the nature of the complementizer.

The study extends the viability of Lardiere’s feature re-assembly account of L2 acquisition to L3 acquisition. The (in)-definiteness of the head noun of the relative clause needs adequate attention in language teaching, like the much-highlighted aspects of resumption.