  A particular three-body mechanism is responsible for the missing strength
which has been reported in $^3$He(e,e$'$p) reactions at missing momentum above
700 MeV/c. It corresponds to the absorption of the virtual photon by a nucleon
at rest which subsequently propagates on-shell and emits a meson which is
reabsorbed by the pair formed by the two other nucleons. Its amplitude, which
is negligible in photon induced reactions as well as in the electro-production
of an on-shell meson, becomes maximal in the quasi-free kinematics (X=1). It
relates the amplitude of the $^3$He(e,e$'$p)D reaction to the amplitude of $pD$
elastic scattering at backward angles.
