To date, most classroom-based research into the learning of a third language (L3) in the US has examined patterns of acquisition among various groups of bilinguals. Situated mainly within the context of "Portuguese for Spanish Speakers" (PSS), studies typically group learners based on the sequence / age of onset of their main background languages, Spanish and English (e.g. Johnson, 2004), revealing differential and oftentimes conflicting results. This raises questions as to the most appropriate way to conceive of individual differences (IDs) within the L3 classroom. This dissertation thus explores how these students differ from one another, and the impact of various IDs on target-language proficiency at the end of a semester of L3 instruction. Participants were 28 Spanish-English bilinguals taking accelerated introductory French, Italian, or Portuguese. By analyzing L3 development over a three-month span, this study first examined the role of prior language experience, encompassing sequence / age of onset, proficiency in the shared pivot language (i.e. Spanish), and classroom exposure to heritage or foreign languages. A series of other IDs included metalinguistic awareness, measured via retrospective written protocols after a brief language-learning task (e.g. Jackson, 2014), as well as motivation, anxiety, strategy use, and aptitude. Target-language domains included L3 lexical and grammatical knowledge, as well as L3 written complexity, accuracy, and fluency. The study yielded several significant correlations, including metalinguistic awareness being tied to classroom language-learning experience, while anxiety correlated differentially with sequence / age of onset. Longitudinal results showed that pivot-language proficiency was predictive of gains in written complexity, while metalinguistic awareness predicted growth in grammatical knowledge. By the semester's end, it was revealed that motivation had decreased and pivot-language proficiency had increased, neither of which was expected. Despite the modest sample size, it can nonetheless be concluded that metalinguistic awareness plays an important role in the L3 classroom and deserves further attention, whereas sequence / age of onset -- though ubiquitous in PSS literature -- does not emerge as the best way to account for prior language experience. Finally, the pedagogical implications of these results are discussed, as well as their contribution to the field of instructed L3 acquisition. [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.]