The decay of a neutron into a proton, an electron, and a neutrino is an example of a three particle.

The decay of a neutron into a proton, an
electron, and a neutrino is an example of a three particle decay process. Use
the vector nature of momentum to show that if the neutron is initially at rest,
the velocity vectors of the three must be coplanar (that is, all in the same
plane). The result is not true for numbers greater than three.