When learning a third language (L3), learners, according to researchers, generally rely on a variety of resources, such as their L1 (first language), L2 (second language), and/or their current knowledge of the L3. Although studies have identified a number of factors that may influence a learner's choice of the source of transfer, these works were mostly cross-sectional in design and took place within a limited time frame. As a result, temporal change in the use of the prior linguistic resources was poorly documented. Using a case study approach, this article examines one hearing learner's L3 acquisition of ASL, specifically, the motion verbs go and walk, as well as the word orders of their argument structures with goal prepositional phrases over a period of one year. The results show that, over time and influenced by several factors, the learner drew both constantly and differentially on his L1, L2, and his current knowledge of the L3.