It is known that Turing systems in two dimensions produce spotted,
striped, and labyrinthine patterns. In three dimensions, a greater
variety of patterns is possible. By numerical simulation of the
FitzHugh--Nagumo type of reaction-diffusion system, we have obtained
not only lamellar, hexagonal and spherical structures (BCC and FCC)
but also gyroid,
Fddd, and perforated lamellar structures. The domains of these three
structures constitute interconnected regular networks, a characteristic
occurring in three dimensions. Moreover, we derive the Lyapunov
functional by reducing the system, and we evaluate this functional by
introducing the asymptotic solutions of each structure by the
mode-expansion method and direct simulation of the time evolution
equation.