In recent years, Third Language Acquisition (TLA) in Cross-Linguistic Influence (CLI) research has been debated regarding background languages' impact on learning a target language. While Second Language Acquisition (SLA) research often emphasizes the primary influence of the first language (L1) on the target language, it does not discount the possibility of other languages, including the second language (L2), having an influence. In contrast, TLA argues that multiple background languages can also transfer into a third language (L3). Results vary due to learner-related factors and methodological differences, and uncertainty remains regarding reliable language transfer models and the significance of learner variability in L3 acquisition. To address these inconsistencies, this study utilized a meta-analysis approach that aggregated effect sizes from 29 peer-reviewed journal articles and two unpublished dissertations/reports from 2010 to 2021, focusing on SLA and TLA. The study then calculated the average correlation between transfer models and proficiency in a third language, aiming to identify the most effective transfer models to help learners overcome the difficulties of learning a third language.Furthermore, the study examined the effects of learner-related factors and different methodological approaches to establish consistency among the study findings. The analysis used a common metric (Hedge's g) with the assistance of R programming software to calculate each study's effect size. The result showed no significant difference in the effect sizes for the ten proposed language transfer models. However, the regression analysis of the moderator variables indicated that individual differences such as language proficiency level and educational setting played a significant role in the TLA process and in determining the learners' target language proficiency at the initial and later stages of learning. The findings can be helpful for curriculum developers and instructors, providing insight into multilingual learners' unique needs and variabilities. Along with this understanding, curriculum developers and instructors can modify their instructional methods to accommodate these differences, ultimately assisting multilingual learners in enhancing their proficiency in the target language. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]