A novel three-dimensional non-destructive beam monitor named π3 was conceived, realized and tested. It is based on a thin aluminum foil coated with P47 scintillating material mounted on a support, together with a miniaturized CCD camera, both moving along the beam axis. This detector allows reconstructing of the beam distribution along the beam path, providing either an on-line video or a graphical reconstruction of the beam envelope in 3D. The π3 detector is a general-purpose instrument suitable for any ion accelerator facility. As it is constructed with non-magnetic materials, it can be used to investigate the behavior of the beam inside beam optics components such as magnets. In this paper, we report the development of the first prototype of the π3 detector, its associated software and the results of the beam tests performed at the Bern medical cyclotron laboratory.