  We investigate systems of three mutually interacting particles with masses of
which the inner is much bigger than the intermediate and the latter is much
bigger than the outer. Then the three-body problem reduces to the two-body
scattering or structure of the light one in the field of the pseudo-nucleus
formed by two others. We calculate analytically the properties of considered
systems, such as the scattering cross-sections, hyperfine splitting, Auger
decay of exited states and Lamb shits, presenting them as expansions in powers
of the ratio of light to intermediate particle masses. This ratio is the small
parameter of the studied problems.
