Theoretical research of a FM-AFM-FM trilayer consisting of superthin layers (with the thicknesses of some atomic monolayers) has been conducted; possible stable spin configurations, the boundaries of their stable regions and their dependencies on different parameters (the value of the external magnetic field, constants of exchange interaction inside the layers and on the boundaries between them, the constant of axial anisotropy) have been found numerically. It is shown that the form and the number of the possible stable spin configurations and the critical values of the magnetic field of the transitions between them dramatically depend on the ratio of exchange interaction values inside the layers and on the boundaries between them. It is shown that the existence of the exchange interaction on the boundary between layers leads to division of the phase of the system which corresponds to the ‘antiferromagnetic’ phase of the bulk antiferromagnet, into two or three different new phases in dependence of the value of the exchange interaction on the boundary.