AC3 stood for Atelier de Creation et de Conception Composite. It was a company established in 1998 in Nice by three graduates from the Sbarro design school. Apart from the Vaillante Grand Defi I think they also made a buggy and Lotus 7 and Porsche 356 updated replicas. Lasttime I heard about them was 2002, so I guess the company doen't exist anymore. Their website also disppeared so that would support it.