  A cosmological model of dark energy interacting with dark matter and another
general component of the universe is considered. The evolution equations for
coincidence parameters r and s, which represent the ratios between the dark
energy and the matter and the other cosmic fluid, respectively, are analyzed in
terms of the stability of stationary solutions. The obtained general results
allow to shed some light on the coincidence problem and in the equations of
state of the three interacting fluids, due to the constraints imposes by the
stability of the solutions. We found that for an interaction proportional to
the sum of the DE density and the third fluid density, the hypothetical fluid
must have positive pressure, which leads naturally to a cosmological scenario
with radiation, unparticle or even some form of warm DM as the third
interacting fluid.
