  We extend our previous study of the quark-hadron phase transition at finite
temperatures with zero net baryon density by two flavor Nambu-Jona-Lasinio
model with Polyakov loop to the three flavor case in a scheme which
incorporates flavor nonet pseudo scalar and scalar mesonic correlations on
equal footing. The role of the axial U(1) breaking Kobayashi-Maskawa-'t Hooft
interaction on the low-lying thermal excitations is examined. At low
temperatures, only mesonic correlations, mainly due to low mass mesonic
collective excitations, pions and kaons, dominate the pressure while thermal
excitations of quarks are suppressed by the Polyakov loop. As temperature
increases, kaons and pions melt into the continuum of quark and anti-quark
excitations successively so that hadronic phase changes continuously to the
quark phase where quark excitations dominate pressure together with gluon
pressure coming from the effective potential for the Polyakov loop. Since we
introduce mesons as not elementary fields but auxiliary fields made from
quarks, we can describe the phase transition between hadronic phase and quark
phase in a unified fashion.
