The Philosopher (1989)

The obscure philosopher Georg almost withdrew from the world to concentrate on his studies, especially Heraclitus, having no relationship since his mother's death. When he wants a new suit for a lecture (actually very minor) about his new book, he meets the sisters Franziska, Breate and Marthe, who run a boutique, share a house and have a lover each. They invite him to dinner, Franziska even on dates. Falling off a boat he gets naked with her, and they become lovers. The girls all take to him and get him to move in, spoiled by all and offered polygamy.