Isotopes are defined as those atoms which have same atomic number but different atomic masses.
Atomic mass is basically the number of protons and neutrons present in an atom.
Atomic number is the number of protons present in an atom.
So, in isotopes the number of protons are same but the number of neutrons vary due to which atomic masses also vary. In given three isotopes, all have same number of protons but different number of neutrons. i.e. H-1 = 1 P + 0 N = 1 u (Proton) H-2 = 1 P + 1 N = 2 u (Deuterium) H-3 = 1 P + 2 N = 3 u (Tritium)
Hence, it is clear that the number after H shows a change in number of neutrons and mass number.
