In my textbook it is written that Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into p + electron + anti-neutrino . If one of the neutrons in Triton decays, it would transform into $He^3$ nucleus. This does not happen and the reason is given to be that both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a $He^3$ nucleus.
But, why is necessary that both the nucleus should decay simultaneously?