  We find and study four kinds of five-parameter family of six-dimensional
coupled Painlev\'e III systems with affine Weyl group symmetry of types
$D_5^{(1)},B_5^{(1)}$ and $D_6^{(2)}$. We show that each system is equivalent
by an explicit birational and symplectic transformation, respectively. We also
show that we characterize each system from the viewpoint of holomorphy.
