We study the behaviour of small, adiabatic, density perturbations in a
three-component
self-gravitating system, embbebed in a static medium using a non-relativistic
approximation.
Using a dynamical systems approach, we show that when the fluid mixture is
perturbed
linearly, under the conditions prevailing in the post-recombination epoch,
besides the existence of exponentially growing and decaying modes that are
present for
values of the perturbation wave-number $k$ smaller than a critical value
$k_M$, there
exists two other pure oscillatory modes at all scales.
It is also shown that when the fluid mixture is perturbed
linearly, under the conditions prevailing in the post-recombination epoch, a
resonant
process occurs in the amplitude of the growing mode of the baryonic system when
we
take a universe mainly composed of Hot Dark Matter.
These results are very similar to those obtained by P.G.Macedo and P.M. de
Carvalho
for a two-component fluid.