  One-nucleon overlap functions, needed for nucleon-removal reaction
calculations, are solutions of an inhomogeneous equation with the source term
defined by the wave functions of the initial and final nuclear states and
interaction between the removed nucleon with the rest. The source term approach
(STA) allows the overlaps with correct asymptotic decrease to be modelled while
using nuclear many-body functions calculated in minimal model spaces. By
properly choosing the removed nucleon interaction the minimum-model-space STA
can reproduce reduced values of spectroscopic factors extracted from
nucleon-removal reactions and predicts isospin asymmetry in the spectroscopic
factor reduction. It is well-known that model space truncation leads to the
appearance of higher-order induced forces, with three-nucleon force being the
most important. In this paper the role of such a force on the source term
calculation is studied. Applications to one-nucleon removal from double-magic
nuclei show that three-nucleon force improves the description of available
phenomenological overlap functions and reduces isospin asymmetry in
spectroscopic factors.
