This paper examines the role of the first (L1, English) and second (L2, Japanese or Spanish) languages in ab initio third language (L3, Latin) acquisition in L2 classroom learners without immersion experience. To present a fuller developmental explanation of how transfer works at the morphosyntactic level, the Competition Model (CM) was used as theoretical framework. Positive shifts in total precision and sentence processing patterns in L3 Latin role assignment demonstrate L3 progress as mainly regulated by the L1, denoting that higher planes of L2 resonance are required for integrated shapes of L1 and L2 cues to appear.