3 Into 2 Won't Go(1969)

The story of a man... a woman... and a girl who rated all her lovers in a little black book!

Steve Howard, a British sales executive living in Manchester, England, begins an affair with a young hitchhiker, Elle Patterson, to emotionally get away from his marriage to his wife Francis. But when Elle moves into a room in Steve and Francis's house, he must keep the true nature of his relationship with Elle under wraps at all costs.