This study examined L3 stops produced by two groups of L3 learners, KEJ speakers (L1 Korean/L2 English/L3 Japanese) and JEK speakers (L1 Japanese/L2 English/L3 Korean), with varying levels of L2 and L3 proficiency. The learners produced L2 and L3 stops in word-initial position, and the VOTs and F0 of their L3 stops were compared to those of monolingual speakers. This study had four main findings. First, JEK speakers produced native-like L3-Korean aspirated stops in terms of VOT when they are advanced L2 speakers. This is known as the positive L2 effect. Second, KEJ speakers proficient in both L2 and L3 produced native-like voiced and voiceless stops in L3-Japanese in terms of VOT. Third, the disparities in outcomes between JEK and KEJ speakers with respect to the L2 effect were due to the typological similarities between L1 and L2. Finally, both JEK and KEJ speakers produced native-like L3 stops in terms of F0 only after acquiring advanced proficiency in both L2 and L3. These findings suggest that the interaction between L2 and L3 proficiency levels and L1 and L2 typological similarities affects the production of L3 stops.