Thermal evolution of ferroelastic domain structure and differential scanning calorimetric (DSC) investigations of Rb3H(SeO4)(2) (TRHSe), CS3H(SeO4)(2) (TCHSe) and (NH4)(3)H(SO4)(2) (TAHS) in the vicinity of the superprotonic phase transition are presented. It was established that new domain structure appears in Rb3H(SeO4)(2) crystals at (T-s -5.8) K, and another fine domain arises at approximate to (T-s -1.8) K. In CS3H(SeO4)(2) and (NH4)(3)H(SO4)(2) crystals, a very dense domain pattern appears at 1.3 and 0.6 K, respectively, below the superprotonic phase transitions. DSC measurements show anomalies at similar temperatures at which changes in the ferroelastic domain pattern were observed. The anomalous behavior of domain structure indicates that the transition to superprotonic phase is more complicated than that described in the literature.