We report on a first-principles supercell calculation of cubic inclusions in 6H-SiC. Motions of successive partial dislocations having the same Burgers vector in the basal planes can lead to a 3C-like region in the perfect 6H-SiC crystal, which corresponds to multiple stacking faults. We have calculated the electronic structures and the total energies of 6H-SiC crystals containing m stacking faults (m=l-4) in the adjacent basal planes, based on the density functional theory in the local density approximation. It has been found that 3C-like sequences in the 6H-host crystals can act as planar quantum wells, in which conduction band electrons can be confined. The total energy calculations using both the supercell method and the axial next nearest neighbor Ising model (ANNNI) have revealed that the 2nd stacking fault energy in 6H-SiC is about 6 times larger than that of an isolated stacking fault.