This study explores how people use and expand their linguistic resources in the situation when they have some proficiency in L2 and try to understand L3 that is related to L2. The focus of the study is on the comprehension of Ukrainian by Estonian L1 speakers via their proficiency in Russian (L2). This situation is labeled as mediated receptive multilingualism. The aim of this research is to investigate the role of cross-linguistic similarity (objective or perceived, in the terms of Ringbom 2007) and extra-linguistic predictors of success in comprehension. In addition to measuring the success rate, we pay attention to the participant's perspective. The experiment was conducted with 30 speakers of Estonian as L1 and included a questionnaire, C-test in Russian, three Ukrainian texts with different groups of tasks, and debriefing. In this article, we focus on the task of defining Ukrainian words from the text and on debriefing interviews. The results showed that similarity, perceived or objective, is not the only decisive factor in facilitating understanding. The participants’ explanations confirmed our previous findings that similarity, albeit important, is only partly responsible for successful comprehension. This became clear from the debriefing interviews. In many cases, the participants' choice was affected by a range of extra-linguistic factors: general knowledge, context, exposure to various registers of Russian, M-factor, meta-linguistic awareness, and learnability. In some instances, context and general knowledge outweighed similarity. These findings show how similarity worked together with extra-linguistic factors in facilitating successful comprehension in challenging multilingual settings.