  We employ the determinant quantum Monte Carlo method to study the
thermodynamic properties of the attractive SU(3) Hubbard model on a honeycomb
lattice. The thermal charge density wave (CDW) transition, the trion formation,
the entropy-temperature relation and the density compressibility are simulated
at half filling. The CDW phase only breaks the lattice inversion symmetry on
the honeycomb lattice and thus can survive at finite temperatures. The
disordered phase is the thermal Dirac semi-metal state in the weak coupling
regime, while in the strong coupling regime it is mainly a Mott-insulated state
in which the CDW order is thermally melted. The formation of trions is greatly
affected by thermal fluctuations. The calculated entropy-temperature relations
exhibit prominent features of the Pomeranchuk effect which is related to the
distribution of trions. Our simulations show that the density compressibility
is still nonzero immediately after the CDW phase appears, but vanishes when the
long-range order of trions forms.
