Since the mid 90s, an increasing number of researchers have adopted a sociocultural theory (SCT) of mind to investigate the social and cognitive functions of language during learner-learner interaction (Lantolf & Thorne, 2007). Researchers from an SCT perspective have identified that first languages (L1s) serve important cognitive functions (Alegría de la Colina & García Mayo, 2009; Storch & Aldosari, 2010). Swain and colleagues (Swain, 1995; Swain & Lapkin, 1995, 1998) have also illustrated that languaging, a form of verbalization, facilitates the completion of complex linguistic tasks which leads to second language (L2) development (Swain, Brooks, & Tocalli-Beller, 2002). Moreover, researchers have found that task type impacts language development (Storch & Aldosari, 2010; Storch & Wigglesworth, 2003; Swain & Lapkin, 2001). Due to the growing number of multilingual learners in the world today (Hammarberg, 2010), researchers need to expand the scope of the research to include the role(s) of native and nonnative languages on third language (L3) development. Thus, the purpose of the current multiple case study was to investigate the specific mediating functions of multilingual learners’ languages during four types of collaborative tasks and to explore the relationship between languaging and L3 development. A 16-week classroom-based study was conducted in a university French as a foreign language classroom in Mexico with four focal participants. The language produced during learner-learner interaction was examined using three types of analysis: (1) each turn was coded for language and for their specific functions; (2) each Language-Related Episode (LRE) was coded for type and for resolution; and (3) accuracy on individual tailor-made posttest items. Findings uncovered a complex picture of task type effects on the specific mediating functions of language as well as complementary functions of L1 and L3 mediation. Results from the analysis of LREs show that task type impacts the occurrence and resolution of LREs. Accuracy scores from the posttests suggest that L1 and L3 mediation promotes L3 development. Findings are in line with the focal participants’ beliefs. The findings that languages serve various social and cognitive functions during task completion are discussed in light of current ideas from an SCT perspective.