A Sand Castle (1962)

Original title: Peščeni grad

Two boys are on an unplanned vacation by foot. They meet a girl who has nothing but a bikini under her coat. She says her cloths were stolen while she was bathing. She joins their company. They steel cloths for her. There is a little kissing with both but nothing that could make the other jealous. During most of the movie they enjoy minor pleasures. But occasionally when the girl is alone she has attacks of very painful emotions. In the end it turns out that she is a former child-prisoner of a German KZ-lager, who had run away from a mental hospital in bikini and coat. Some of the staff were searching for her. But when she sees them she commits suicide.