  In the tight-binding approximation the Harper like equation describing an
electron in 3D crystal subject to a uniform magnetic field is obtained. It is
supposed that the vector H can be oriented along several directions in the
lattice. The Fermi surfaces relevant to a magnetic flux p/q=1/2 in a simple
cubic lattice are built. The quantization rules in magnetic fields slightly
distinguished from p/q=1/2 are investigated.
