Since you specified no combination of protons and neutrons can be stable at that number, it's not three.Helium-3 is perfectly stable. The answer is 5. Hydrogen-5 is highly unstable and decays by double neutron emission, with a half-life of around [math]10^{-21}[/math] seconds. read more
Only 13 of the 38 known-but-unstable elements (assuming the total number of elements is 118) have isotopes with a half-life of at least 100 years. Every known isotope of the remaining 25 elements is highly radioactive; these are used in academic research and sometimes in industry and medicine. read more