Most speakers' first language (L1) in Hong Kong and Guangdong China is Cantonese. Even though the L1 of both Guangdong Cantonese speakers and Hong Kong Cantonese speakers is the same, their second languages (L2) belong to different language families. Previous studies have pointed out that L2 status in third language (L3) acquisition is a salient factor. This study aimed to: (1) examine Guangdong Cantonese speakers' pronunciation error patterns in their L2 Mandarin and L3 English and explore the language interaction patterns among these three languages; (2) compare the findings with those of Hong Kong Cantonese speakers whose L2 is English and L3 Mandarin in Chen and Han (2019. L3 phonology: contributions of L1 and L2 to L3 pronunciation learning by Hong Kong speakers. International Journal of Multilingualism, 16(4), 492-512. https://doi.org/10.1080/14790718.2019.1573901) to identify the roles of L1 and L2 in L3 acquisition. Results of the reading aloud tasks, a questionnaire survey and interview data show that the language interaction patterns among L1, L2 and L3 are influenced by learners' language proficiency.