  We present a new method based on the scattering technique to investigate
fluctuation-induced forces at a fluid interface. The scattering approach, well
suited to the study of many body systems of arbitrary geometries, is augmented
to include boundary fluctuations. Using this method, we study the deviation of
the total fluctuation-induced interaction from the sum of pairwise energies for
three colloidal particles. We consider both frozen and fluctuating colloids and
obtain a very good agreement between analytical and numerical results. We find
a marked difference in the three body fluctuation-induced free energy between
the frozen and fluctuating colloids, both in sign and relative size.
