This paper addresses the acquisition of three English syntax–discourse interface constructions: Focus Fronting, Left Dislocation and Topicalization. Data from Basque–Spanish bilinguals acquiring English as a third language serve as a test case for the Interface Hypothesis (IH, Sorace, 2011). Findings offer evidence against modern L3 acquisition models and possible arguments for both cumulative improvement and cumulative impediment as potential L3 acquisition processes.