What are three isotopes of H?
Protium, Deuterium, & Tritium.
To enlighten further.
#1; Protium ; 1 proton, 0 neutrons, 1 electron ; Atomic mass '1'.
#2; Deuterium ; 1 proton, 1 neutron, 1 electron ; Atomic mass '2'.
#3 ; Tritium ; 1 proton, 2 neutrons, 1 electron ; Atomic mass '3'.
NB. Protium is the most common isotope of hydrogen, and casually just referred to as 'hydrogen'.
Deuterium is much less common isotope of hydrogen , and casually referred to as 'heavy hydrogen'. Symbolically it is 'D'.
Tritium is the rarest isotope of hydrogen , sometimes referred to as 'super-heavy; hydrogen. It is also radio-active. Symbolically it is 'T'.
Notice it is ONLY the neutrons number that varies, and consequently the atomic mass.
Remember definitively 'An isotope has a different number of neutrons'.