  Asymptotically AdS rotating black holes for the Bergshoeff-Hohm-Townsend
(BHT) massive gravity theory in three dimensions are considered. In the special
case when the theory admits a unique maximally symmetric solution, apart from
the mass and the angular momentum, the black hole is described by an
independent "gravitational hair" parameter, which provides a negative lower
bound for the mass. This bound is saturated at the extremal case and, since the
temperature and the semiclassical entropy vanish, it is naturally regarded as
the ground state. The absence of a global charge associated with the
gravitational hair parameter reflects through the first law of thermodynamics
in the fact that the variation of this parameter can be consistently reabsorbed
by a shift of the global charges, giving further support to consider the
extremal case as the ground state. The rotating black hole fits within relaxed
asymptotic conditions as compared with the ones of Brown and Henneaux, such
that they are invariant under the standard asymptotic symmetries spanned by two
copies of the Virasoro generators, and the algebra of the conserved charges
acquires a central extension. Then it is shown that Strominger's holographic
computation for general relativity can also be extended to the BHT theory;
i.e., assuming that the quantum theory could be consistently described by a
dual conformal field theory at the boundary, the black hole entropy can be
microscopically computed from the asymptotic growth of the number of states
according to Cardy's formula, in exact agreement with the semiclassical result.
