³He has atomic number 2; the nucleus has 2 protons and 1 neutron. ²H has atomic number 1; the nucleus has 1 proton and 1 neutron. Combined, there are 3 protons and 2 neutrons. If an alpha particle is formed, it is ⁴₂He; it has 2 protons and 2 neutrons. So all that is left over is 1 proton.