This paper reports on an empirical investigation of how knowledge of L2 Russian can facilitate the acquisition of passive knowledge of L3 Ukrainian by speakers of L1 Estonian. The experiment was conducted with 30 speakers of Estonian as L1, who first filled in a sociolinguistic questionnaire, then completed a C-test in their L2 Russian, before carrying out a task testing their understanding of L3 Ukrainian words and texts, and providing some feedback in a debriefing session. We pay specific attention to the performance on the Russian C-test and how participants' scores correlate with their results on the Ukrainian tasks. We also made an inventory of the grammatical and lexical elements that proved easy or difficult. The results show a positive correlation between the scores on the C-test and performance on the Ukrainian tasks. However, this correlation was lower for text understanding in Ukrainian than for understanding separate Ukrainian words. This suggests that a C-test score does not predict participants' ability to understand the Ukrainian texts to a full extent, while it has better predictive value for the understanding of individual Ukrainian words. These findings suggest that learners use resources beyond just L2 lexical-grammatical knowledge in forming an understanding of texts in an L3 that is closely related to the L2.