  The effective dc-conductivity problem of isotropic, two-dimensional (2D),
three-component, symmetric, regular composites is considered. A simple cubic
equation with one free parameter for $\sigma_{e}(\sigma_1,\sigma_2,\sigma_3)$
is suggested whose solutions automatically have all the exactly known
properties of that function. Numerical calculations on four different
symmetric, isotropic, 2D, three-component, regular structures show a
non-universal behavior of $\sigma_{e}(\sigma_1,\sigma_2,\sigma_3)$ with an
essential dependence on micro-structural details, in contrast with the
analogous two-component problem. The applicability of the cubic equation to
these structures is discussed. An extension of that equation to the description
of other types of 2D three-component structures is suggested, including the
case of random structures.
  Pacs: 72.15.Eb, 72.80.Tm, 61.50.Ah
