And coming even later with a reply - two characters named Sanxi appear in "The Return of
Martin Guerre" by
Natalie Zemon Davies. As the book is about a true story, written basing on a documents, I would consider it a trustworthy source of information. Both Sanxis there are of Basque origin (a grandfather and his grandson), living in 16th century
France, but sadly there is no information on the meaning of the name.