08-07U

The scheme for obtaining the resulting functional for the partition function of the $3D$ model of magnet in the external field with using of the set of collective variables (CV) is represented. It is shown, that the use of series of approximations for the Jacobian of the transition to the CV generates the chain of the various approximate methods for describing the phase transition: the mean-field theory, the Gauss fluctuations theory, the Landau approach, etc. The conditions of the approximate description of the $3D$ system behaviour near the critical point are formulated without using any assumptions of phenomenological character and involving any macroscopic parameters (for example, the order parameter).