Some specific features of the microphase separation in molten ternary ABC triblock copolymers (in particular, modifications of the molten AB diblock copolymers by attaching a short third block C strongly incompatible with both A and B blocks) are studied rigorously within the framework of the weak segragation approach. It is shown that via such a modification one can control both the stability as to the microphase separation transition (i.e., the corresponding spinodal temparatures) and the smoothness of such a transition. Two modes of such modifications for every composition are studied in detail : (i) those resulting in the minimal spinodal temperatures and (ii) those that result in the smooth order-disorder transition at this composition. The corresponding dependencies of the transition temperature and the periodicity of the occurring supercrystal structures are built and analyzed.