Coupled Painlev\'e III system with affine Weyl group symmetry of type $D_6^{(1)}$

We find and study a six-parameter family of coupled Painlev'e III systems in dimension six with affine Weyl group symmetry of type $D_6^{(1)}$. We also find and study its degenerate systems with affine Weyl group symmetry of types $B_5^{(1)}$ and $D_5^{(2)}$.