Tritium ($^3_1H$) is a radioactive isotope of hydrogen containing 1 proton and 2 neutrons. It undergoes beta decay: $^3_1H \to ^3_2He + \beta^- + \bar{\nu}_e$. The emitted $\beta^-$ particles have very low energy (~18 keV max). Protium and deuterium are stable; $H^+$ is simply a proton with no radioactivity.