Tritium is an isotope of hydrogen whose nucleus Triton contains 2 neutrons and 1 proton. Free neutrons decay into $\mathrm{p}+\overline{\mathrm{e}}+\overline{\mathrm{\upsilon }}$. If one of the neutrons in Triton decays, it would transform into He³ nucleus. This does not happen. This is because

both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a  nucleus.

the electron created in the beta decay process cannot remain in the nucleus

both the neutrons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not a He³ nucleus.

Triton energy is less than that of a He³ nucleus.