The self-consistent theory of the magnetotail is extended from two dimensions to three. An asymptotic solution for small variation of the field quantities in the direction perpendicular to the normal of the neutral sheet is obtained in closed form. An attempt is made to match the remaining parameters to observational data. The theory contains a number of structural properties of the tail that were not available in the earlier two-dimensional model, such as the solar magnetosphere y component of the magnetic field, tail flaring and plasma sheeet variation in the y direction, and field-aligned currents.