A colourless, odourless, tasteless, flammable gaseous substance that is the simplest member of the family of chemical elements. The hydrogen atomhas a nucleus consisting of a proton bearing one unit of positive electrical charge; 
Hence the three isotopes.
Protium, neutron and deuterium are chemically alike because Hydrogen: physical properties. Under ambient conditions, hydrogen is a colourless highly flammable diatomic gas with the molecular formula H2. Hydrogen gas has no small or taste. It is possible to make liquid hydrogen at atmospheric pressure by cooling it to below 20.28 K (-252.87 °C).. And with one proton and two neutrons in each nucleus, is the mass 3 isotope and constitutes about 10−15 to 10−16 percent of hydrogen. The practice of giving distinct names to the hydrogen isotopes is justified by the fact that there are significant differences in their properties.