The theory of nuclear matter is modified in such a way that the concept of the one-body potential is given up for the off-energy shell and kept only for the on-energy shell. In this method no longer occur such complications as many-particle excitations which couple one by one through the off-energy shell potential and we treat all higher order terms with a given number of hole lines as a whole. Following this method the two-body and three-body contributions to the binding energy are calculated for the hard core and soft core potentials. The two-body contributions are about 10 MeV for the soft core and 5 to 7 MeV for the hard core potentials. The three-body contributions is found to be sensitive to the behavior of the two-body correlation function in the outer healing region. The three-body contributions are repulsive and about 0.5 MeV at r0=1.1 fm for the soft core and 0.8 MeV at r0=1.6 fm for the H-J potential.