In three-wave VFEL we consider three strong electromagnetic waves that are under diffraction conditions. There are three possible geometries in such a system. First: so-called Bragg-Bragg case, when one wave goes forward and two waves go in the opposite direction. Second: Bragg-Laue case, when two waves go forward and one wave goes in the opposite direction. And in the last case, so-called Laue geometry, all three waves spread in one direction. Accordingly to this, we have to write out boundary conditions on different boundaries of the system and use right or left finite difference derivatives in accordance with wave spread directions.

System of equations and boundary conditions for common case of n-wave coplanar distributed feedback geometry were formulated and are discussed too.