Atoms of specific elements have a fixed number of protons and electrons, however, they can have different amounts of neutrons. When atoms have an uncommon number of neutrons, they are called isotopes. Isotopes that are unstable can emit radiation.
An atom of helium most commonly has 2 protons, 2 electrons, and 2 neutrons. Each proton and neutron has a mass of 1amu. If you add the mass of 2 protons and 2 neutrons you can identify that helium usually has a mass of 4amu.
When helium has a mass of 3amu it has fewer neutrons than usual. Since helium atoms always have 2 protons, the helium-3 isotope of helium has 1 neutron.