The overall aim of the present study is to achieve a better understanding of the role played by previous languages for word knowledge in a third language (L3). More specifically, the study examines cognate effects in vocabulary size estimates in Swedish learners of French L3, who all have English as their second language (L2). In a previous study (Lindqvist 2018), it was found that these learners, pupils in grades 6, 7, 8, and 9 in the Swedish school system (aged 12–15), achieved relatively high vocabulary size scores as measured by the X-Lex test. This result led to the assumption that the learners were affected by, and possibly took advantage of, their knowledge of English L2, resulting in higher scores than expected. A detailed analysis of the correct answers on the test is conducted in the present study. The results show that the majority of the correct answers are French/English/Swedish cognates, thus confirming the expectations.