Specific heat of PbMg1/3Nb2/3O3, PbMg1/3Ta2/3O3, and BaMg1/3Ta2/3O3 complex perovskites has been measured from 2 K up to 295 K. At low temperatures a drastic difference of the behaviour of the specific heat of BaMg1/3 Ta2/3O3 with respect to other two compounds was found. Different approaches to describe the specific heat at the low temperatures of the PbMg1/3Nb2/3O3 relaxor ferroelectric are discussed.