While formal L3 morphosyntactic acquisition research has focused on the roles of the L1 versus L2 during the L3 initial stages, we examine their roles during development. Specifically, we explore whether the L3 (here, Portuguese) revision process after non-facilitative transfer differs whether the initial L3 representation reflects the L1 or L2. If this initial L3 representation is the same for a group whose source is their L1 and a group whose source is their L2, it is plausible to predict similar developmental processes between groups. However, recent data have shown between-group developmental differences despite a similar L3 starting point. In an effort to isolate the variable(s) behind such differences, we investigate the roles of dominance and formal instruction via two sets of L3 morphosyntactic judgments. The first set excludes dominance, with similar patterns in English-dominant versus Spanish-dominant L1 Spanish data. The second set excludes formal instruction, whereby L2 Spanish data shows L3 convergence for a property not explicitly taught and L1 Spanish data does not. As an initial hypothesis for further exploration of between-group differences, we propose the Cumulative Input Threshold Hypothesis: The rate of developmental recovery from non-facilitative transfer is inversely related to lifetime input of the source language.