The new equation of raindrop-size distribution will be proposed in this paper. It may be represented by the normal distribution of liquid water content per unit volume of air with size. Almost of the conventional distribution (e.g. Marshall-Palmer, and Best equation) and the observed and computed stationary distribution (reported in Part 1 and Part 2 of this paper) are to obey to this rule. Some physical considerations on such a rule will be made and the characteristics of the derived rain parameters will be described for radar meteorology.