  We show that the contributions of three-quasiparticle interactions to normal
Fermi systems at low energies and temperatures are suppressed by n_q/n compared
to two-body interactions, where n_q is the density of excited or added
quasiparticles and n is the ground-state density. For finite Fermi systems,
three-quasiparticle contributions are suppressed by the corresponding ratio of
particle numbers N_q/N. This is illustrated for polarons in strongly
interacting spin-polarized Fermi gases and for valence neutrons in neutron-rich
calcium isotopes.
