  We develop a theoretical approach for nuclear spectral functions at high
missing momenta and removal energies based on the multi-nucleon short-range
correlation~(SRC) model. The approach is based on the effective Feynman
diagrammatic method which allows to account for the relativistic effects
important in the SRC domain. In addition to two-nucleon SRC with center of mass
motion we derive also the contribution of three-nucleon SRCs to the nuclear
spectral functions. The latter is modeled based on the assumption that 3N SRCs
are a product of two sequential short range NN interactions. This approach
allowed us to express the 3N SRC part of the nuclear spectral function as a
convolution of two NN SRCs. Thus the knowledge of 2N SRCs allows us to model
both two- and three-nucleon SRC contributions to the spectral function. The
derivations of the spectral functions are based on the two theoretical
frameworks in evaluating covariant Feynman diagrams: In the first, referred as
virtual nucleon approximation, we reduce Feynman diagrams to the time ordered
noncovariant diagrams by evaluating nucleon spectators in the SRC at their
positive energy poles, neglecting explicitly the contribution from vacuum
diagrams. In the second approach, referred as light-front approximation, we
formulate the boost invariant nuclear spectral function in the light-front
reference frame in which case the vacuum diagrams are generally suppressed and
the bound nucleon is described by its light-cone variables such as momentum
fraction, transverse momentum and invariant mass.
