The structural-alpha <--> gamma-transformation and the magnetic properties of a series of Fe-Co-Ni alloys, with pseudobinary type (Fe(x)Ni1-x)(y)Co100-y, were studied as a function of temperature, composition and heat treatment. The measurements were carried out in the temperature range from room temperature up to the magnetic transition temperature of each alloy, and were followed by a calculation of the starting temperature of the alpha <--> gamma-transformation, where this transformation exists, and by a calculation of the mean magnetic moment per atom, the spontaneous magnetization at absolute zero and the Curie point of all alloys studied. Diagrams with the magnetic and structural state, of all alloys studied, as a function of temperature and composition, during temperature increase, are presented. There are indications for the existence of an ordered phase in the alloy which is the compositionally limiting case between transformed and not transformed alloys.