Due to the growing number of plurilingual learners in the world today (Hammarberg, 2010), the present multiple case study examines four plurilingual participants' beliefs toward first language (L1) and second language (L2) mediation in the acquisition of French as a third language (L3). During a 16-week classroom-based study in a French university language class in central Mexico, four Spanish (L1)-English (L2) participants completed eight collaborative pedagogic tasks and participated in four in-depth one-on-one interviews. A qualitative analysis of the focal participants' beliefs about L1 and L2 mediation in the acquisition of an L3 was conducted and their beliefs were compared to task performance gathered from task-based learner-learner interaction. Findings suggest that both native and non-native languages serve cognitive and social functions during task completion but that individual differences are subject to previous factors including educational experiences, language proficiency, and context of language learning.