Solution:

3-isotopes are:

(i) tritium ${ }_{1}^{3} H$ or $\mathrm{T}$

(ii) protium ${ }_{1}^{3} H$

(iii) deuterium ${ }_{1}^{2} H$ or $\mathrm{D}$

Mass Ratio:

Tritium : Protium : deuterium $=1: 2: 3$