Fig. 3 shows a 3D-Mouse.
3D-mice (or similar devices like 3D-joysticks) are the most constrained input devices available for the VR. While the device itself has six degrees of freedom of motion (movement along the 3 axes and 3 degrees of freedom for rotation), its buttons have just one: they are either pressed or released. This constraint allows precise input to the system. It may not be as natural as a gesture or a voice-command, but it is accurate, and that is what is needed when interacting in the VR.
3D mice are usually tracked in 3D space. In effect, then, a 3D cursor is created. This produces another constraint problem, in the sense that the input with these devices will be inprecise (moving an object by moving the hand). Since VR is by nature 3 dimensional, the (physical) motion of the device cannot be constrained to 2 dimensions.
As we see later the input devices, whatever that will be, cannot be constrained by hardware the device must be constrained by software means.