Out of the given options, only option 3, i.e., souring of milk is an example of chemical change. Chemical changes are those changes in which one or more substances are transformed into a new substance with different physical and chemical properties. In the case of souring of milk, the lactic acid bacteria present in the milk metabolize the lactose sugar in the milk, which leads to the formation of lactic acid. This change is irreversible and results in a new substance with different properties than milk.

On the other hand, crystallization of sodium chloride and melting of ice are examples of physical changes. In these changes, there is no formation of new substances with different chemical properties. In the case of crystallization of sodium chloride, the solid sodium chloride is simply separated from the solution, and in the case of melting of ice, the solid ice is converted into liquid water without any change in the chemical composition of the substance.