Solutions
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Ziv Grosu's post in What is the language string for "user, user and user" liked was marked as the answerGo to languages > look for: _list_format_ string. The original sentence is a, b and c , it has to be a translated to a, b yourword c
Put another word instead of "yourword" something that links to the "third" member. now it's working!
This has solved my problem.