  We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the
electronic structure, exchange interactions and Curie temperature in (GaMn)As
and extend the study to two other diluted magnetic semiconductors (GaCr)As and
(GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%,
6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar
features. Both are semi-metallic and ferromagnetic, with similar properties of
the interatomic exchange interactions and the same scale of the Curie
temperature. In both systems the presence of the charge carriers is crucial for
establishing the ferromagnetic order. An important difference between two
systems is in the character of the dependence on the variation of the number of
carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the
presence of the donor defects, like As$_{\rm Ga}$ antisites. On the other hand,
the Curie temperature of (GaCr)As depends rather weakly on the presence of this
type of defects but decreases strongly with decreasing number of electrons. We
find the exchange interactions between 3d atoms that make a major contribution
into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path
responsible for these interactions. The properties of (GaFe)As are found to
differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does
not show a trend to ferromagnetism and is not half-metallic that makes this
system unsuitable for the use in spintronic semiconductor devices.
