In a picture naming study, we examined cross-language activation during speech production in three groups of trilinguals: L3-immersed German–English–Dutch, non-L3-immersed Dutch–English–German, and L3-immersed Russian–English–German trilinguals. All trilinguals named pictures with cognate and non-cognate names in their L2 and their L3. Specifically, we examined cognate effects in same-script trilinguals who were either immersed or not immersed in their L3 and trilinguals whose first language (Russian) differs in script from their other two languages (German, English) to address the questions (1) whether, as non-target language knowledge is co-activated, cognate effects accrue across languages during word production, and (2) whether immersion in L3 is a modulating factor in cross-language activation. We found cognate facilitation in the same-script trilinguals across all languages, although with patterns modulated by the trilinguals’ L3-immersion status and L3 proficiency, corroborating and extending earlier findings in bilingual adults and children. Critically, we also found cognate effects in the different-script trilinguals when the pictures had cognate names in all three languages, indicating that the L1 Russian phonology was activated during naming in L2 English when L3 German was also present, and vice versa.