There has been increasing L3 research on L3 transfer selectivity, especially concerning which factor triggers crosslinguistic influence during L3 acquisition. According to Slabakova's (2016) Scalpel Model, every linguistic property has distinct and unique input factors regarding L3 acquisition such as high/low frequency or positive/negative input. Prior research on L3 has considerably enhanced the understanding of the impact of cognitive and psychological factors, such as the order of language acquisition, typological similarities between L1 and L3 or L2 and L3, and the structural characteristics of the features involved in L3 acquisition. Nevertheless, only a few studies focused on the significance of input on L3 acquisition. In this regard, this dissertation explores whether input, particularly the length of residence, influences parsing preference of ambiguous relative clauses (RCs) (e.g., I met the king of the servant who rules the kingdom) in L3 acquisition. Rather than investigating the unidirectional crosslinguistic influence as in L1 or/and L2 to L3, this dissertation investigates multidirectional crosslinguistic influences, specifically targeting the interaction between non-native languages, L2 to L3 and L3 to L2. This is because earlier L3 studies have predominantly addressed one-way transfer selectivity, creating a gap in the domain of multidirectional aspects of crosslinguistic influence in L3 acquisition. Accordingly, this dissertation examines the impact of residence in a country where a non-native language, either L2 or L3, is spoken, by using both offline timed comprehension and online self-paced reading tasks. The data gathered from the offline timed comprehension tasks carried out by L1- Mandarin speakers who have learned both English and Korean as L2 or L3 with varying length of Korean residence, mainly quantified by the number of years, indicates that Korean residence significantly influences their non-native (L2 or L3) English and Korean resolution of ambiguous RCs. However, results from online self-paced reading tasks did not reveal a significant impact from Korean residence. Thus, the overall results emphasize the substantial role of living in a country where a non-native language is spoken, particularly in offline parsing scenarios, while its impact on online parsing appears limited. It was also revealed that controlling and measuring language input in a L3 context demands more refined and specific methods compared to a L2 context. Overall, this dissertation presents a compelling case for emphasizing the role of input in L3 acquisition, aligning with the Scalpel Model. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]