Aims and Objectives: The present study explores the question of whether learning a third language (L3) in an English as a foreign language (EFL) classroom setting induces improved inhibitory control compared with that found in bilinguals, considering task complexity and language proficiency. Methodology: Thirty-six Chinese-English second language (L2) young adult learners and 121 Chinese-English-Japanese/French/Russian/German L3 young adult learners with three levels of L3 proficiency participated in the study. Simon arrow tasks were employed to measure two types of inhibitory control: response inhibition (the less complex task with univalent stimuli) and interference suppression (the more complex task with bivalent stimuli). Data and Analysis: Statistics using ANOVAs and multiple comparisons were employed to analyze the effects of L3 learning on the reaction time and accuracy for response inhibition and interference suppression, respectively. Findings: The results demonstrated that L3 learners did not outperform L2 learners in the two types of inhibitory control: response inhibition (less complex) and interference suppression (more complex). Moreover, L3 learners with a higher proficiency did not display better inhibitory control than those with a lower proficiency in response inhibition and interference suppression. However, as the L3 proficiency increased, some specific aspects of inhibitory control did improve and exhibited a nonlinear pattern. Originality: The present study extends bilingual advantage in inhibitory control to formal L3 learning, exploring whether bilingual advantage in inhibitory control also appears in L3 learners, considering task complexity and language proficiency. Significance/implications: The present study contributes to the theory of the relationship between multilingualism and inhibitory control by showing that this relationship may be more complex than it is understood currently. Learning an additional language to L2, particularly short-term learning, may not lead to an incremental advantage in overall inhibitory control. However, as learning time increases, changes may appear in specific aspects of inhibitory control, and may be a nonlinear one.