This article considers the acquisition of three English syntax-discourse interface constructions: Topicalization, Focus Fronting and Left Dislocation. We use data from Basque-Spanish bilinguals learning English as a third language (L3) as a test case for the Interface Hypothesis (IH, Sorace, 2011). The IH has made specific predictions about second language (L2) acquisition and such predictions can be extrapolated to L3 on the basis of interface delay explanations. Thirty contexts and embedded test sentences with and without pronouns were used; participants had to rate the acceptability of each audio stimulus sentence in the context on a 7-point scale. We tested Basque/Spanish bilinguals dominant in Basque (n= 23), Basque/Spanish bilinguals dominant in Spanish (n= 24), Spanish L2 English learners (n= 39) as well as native English speakers (n=24). Findings provide evidence against current models and potential arguments for both cumulative enhancement as well as cumulative inhibition as possible processes in L3 acquisition.