Relative atomic mass (or atomic weight) is the average mass of an atom of an element, taking into account the different isotopes and their abundances. Isotopes are atoms of the same element that have different numbers of neutrons.

In this case, we are comparing three isotopes of hydrogen: protium, deuterium, and tritium.

1. Protium:
- Protium is the most common and lightest isotope of hydrogen.
- It consists of one proton and one electron, with no neutrons.
- It has a relative atomic mass of approximately 1.0078 atomic mass units (amu).

2. Deuterium:
- Deuterium is an isotope of hydrogen that contains one proton, one electron, and one neutron.
- It is twice as heavy as protium.
- It has a relative atomic mass of approximately 2.0141 amu.

3. Tritium:
- Tritium is another isotope of hydrogen that contains one proton, one electron, and two neutrons.
- It is the heaviest of the three isotopes of hydrogen.
- It has a relative atomic mass of approximately 3.016 amu.

From the information above, we can see that tritium has the greatest relative atomic mass among the three isotopes of hydrogen. It is heavier than deuterium and protium due to the presence of two neutrons. Protium, being the lightest isotope, has the lowest relative atomic mass.

Therefore, the correct answer is option 'A' - tritium.