  Three-dimensional (3D) $q$-state Potts models ($q$=3, 4, and 5) are studied
by the tensor product variational approach (TPVA), which is a recently
developed variational method for 3D classical lattice models. The variational
state is given by a two-dimensional (2D) product of local factors, and is
improved by way of self-consistent calculations assisted by the corner transfer
matrix renormalization group (CTMRG). It should be noted that no a priori
condition is imposed for the local factor. Transition temperatures and latent
heats are calculated from the observations of thermodynamic functions in both
ordered and disordered phases.
