The burgeoning field of third language (L3) acquisition has increasingly focused on intermediate stages of language development, aiming to establish the groundwork for comprehensive models of L3 learning that encompass the entire developmental sequence. This article underscores the importance of a robust epistemological foundation, advocating for incremental knowledge building through longitudinal research. In the study presented here, we use artificial languages to investigate L3 acquisition from initial exposure with complete input control, factoring in individual differences in executive functions and history of bilingual exposure/engagement to assess the role of these variables in shaping learning trajectories and modulating cross-linguistic influence (CLI). This approach not only advances our understanding of L3 development under controlled conditions but also links L3 acquisition research to broader cognitive science inquiries.