  The equilibrium structure and the dispersion relations of collective
excitations in trilayer Yukawa systems in the strongly coupled liquid regime
are examined. The equilibrium correlations reveal a variety of structures in
the liquid phase, reminiscent of the corresponding structures in the solid
phase. At small layer separation substitutional disorder becomes the governing
feature. Theoretical dispersion relations are obtained by applying the
Quasi-Localised Charge Approximation (QLCA) formalism, while numerical data are
generated by micro-canonical molecular dynamics (MD) simulations. The
dispersions and polarizations of the collective excitations obtained through
both of these methods are compared and discussed in detail. We find that the
QLCA method is, in general, very satisfactory, but that there are phenomena not
covered by the QLCA. In particular, by analyzing the dynamical longitudinal and
transverse current fluctuation spectra we discover the existence of a novel
structure, not related to the collective mode spectra. This also provides a new
insight into the long-standing problem of the gap frequency discrepancy,
observed in strongly coupled layered systems in earlier studies.
