  Macro properties of cold atomic gases are driven by few-body correlations,
even if the gas has thousands of particles. Quantum systems composed of two and
three particles with attractive zero\=/range pairwise interactions are
considered for general masses and interaction strengths in two and three
dimensions (2D and 3D). The Faddeev decomposition is used to derive the
equations for the bound state, which is the starting point for the
investigation of universal properties of few\=/body systems, i.e. those that
all potentials with the same physics at low energy are able to describe in a
model\=/independent form. In 2D, the number of bound states in a three\=/body
system increases without bound as the mass of one particle becomes much lighter
than the other two. The analytic form of an effective potential between the
heavy particles explains the mass\=/dependence on the number of bound energy
levels. An exact analytic expression for the large\=/momentum asymptotic
behaviour of the spectator function in the Faddeev equation is presented. The
spectator function and its asymptotic form define the two- and three\=/body
contact parameters. The two\=/body parameter is found to be independent of the
quantum state in some specific 2D systems. The 2D and 3D momentum distributions
have a distinct sub\=/leading form whereas the 3D term depends on the mass of
the particles. A model that interpolates between 2D and 3D is proposed and a
sharp transition in the energy spectrum of three-body systems is found.
