Salt to the Sea is a 2016 historical fiction young adult novel by Ruta Sepetys. It tells the story of four individuals in World War II who make their way to the ill-fated MV "Wilhelm Gustloff". The story also touches on the disappearance of The Amber Room, a work of art stolen by the Nazis that has never been recovered.
Salt to the Sea has three main people as the characters.
It has four individuals that are the main characters, not three. I think it was worded in a context that threw off the system.