How will you justify that ice, water and steam are the three states of a substance and not different substances?

Because the three states are composed by H2O molecules. If these substances were different they would be composed by different elements. 

The atomic nucleus made up from protons (yellow) and neutrons (blue) and held together by nuclear forces. nuclear forces (also known as nuclear interactions or strong forces) are the forces that act between two or more nucleons. they bind protons and neutrons (“nucleons”) into atomic