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.
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