To which of the isotopes of the given elements only one position is given

Concept: Isotopes of an element have different atomic masses but the same atomic number. Since the periodic table has been framed on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes of a particular element.

Three different positions should have been allotted to the isotopes of hydrogen with atomic masses 1, 2 and 3. But they have been assigned only one position.