The discoveries of electrons, protons and neutrons were made by J. J. Thomson, Ernest Rutherford and James Chadwick respectively. They were staggered over a period of 35 years.
J. J. Thomson discovered the electron in 1897 through his studies of cathode rays, which he found were composed of negatively charged particles much smaller than atoms. In 1911, Ernest Rutherford suggested that atoms also contain a central positive charge, making up most of their mass. In 1932, James Chadwick was able to calculate the mass (and prove the existence) of neutrons by firing them at paraffin wax and determining the energy of the protons they displaced.