A kinetic model of nucleation propagation and fusion ofnumerous cracks is derived. The population of three categories of cracks (shear, tensile and shear-tensile) is taken into account. The exponential from of size crack distributions is deduced for constant fusion cross-sections. This is in agreement with the kinetic model for single category of cracks. A set of three simple differential equations for numbers of cracks in each category is obtained and analysed numerically.