This article reports on a study investigating the relative influence of the first language and dominant language (L1) on second language (L2) and third language (L3) morpho-lexical processing. A lexical decision task compared the responses to English NV-er compounds (e.g. taxi driver) and non-compounds provided by a group of native speakers and three groups of learners at various levels of English proficiency: L1 Spanish – L2 English sequential bilinguals and two groups of early Spanish–Basque bilinguals with English as their L3. Crucially, the two trilingual groups differed in their first and dominant language (i.e. L1 Spanish – L2 Basque vs. L1 Basque – L2 Spanish). Our materials exploit an (a)symmetry between these languages: while Basque and English pattern together in the basic structure of (productive) NV-er compounds, Spanish presents a construction that differs in directionality as well as inflection of the verbal element (V_[3SG] + N). Results show between and within group differences in accuracy and response times that may be ascribable to two factors besides proficiency: the number of languages spoken by a given participant and their dominant language. An examination of response bias reveals an influence of the participants’ first and dominant language on the processing of NV-er compounds. Our data suggest that morphological information in the non-native lexicon may extend beyond morphemic structure and that, similarly to bilingualism, there are costs to sequential multilingualism in lexical retrieval.