Finite groups with a $D$-block of cardinality 3

The results of this paper are connected with one of the main problems of the representation theory, the problem of dependence between the character table of a finite group and its abstract structure. Finite groups that have a $D$-block of cardinality 3 for some normal subset $D$ are investigated. Some properties of classical $p$-blocks (where $p$ is a prime number) consisting of three irreducible characters are also obtained.