  The idea of raising Tc in the spin-fluctuation mediated superconductivity on
disconnected Fermi surfaces with the gap function changing sign across but not
within the Fermi pockets, proposed by Kuroki and Arita for two dimensions (2D),
is here extended to three-dimensional (3D) systems. Two typical cases of 3D
disconnected Fermi surfaces (stacked bond-alternating lattice and stacked
ladder layers) are considered. By solving Eliashberg's equation for Green's
function obtained with the fluctuation exchange approximation (FLEX) for the
repulsive Hubbard model on these structures, we have shown that Tc can indeed
reach O(0.01t), which is almost an order of magnitude higher than in ordinary
3D cases and similar to those for the best case found in 2D. The key factor
found here for the favorable condition for the superconductivity on
disconnected Fermi surfaces is that the system should be quasi-low dimensional,
and the peak in the spin susceptibility should be appropriately "blurred".
