  The principal focus of the present work concerns the critical behaviors of a
class of three dimensional black holes with a scalar field hair. Since the
cosmological constant is viewed as a thermodynamic pressure and its conjugate
quantity as a volume, we examine such properties in terms of two parameters B
and a. The latters are related to the scalar field and the angular momentum
respectively. In particular, we give the equation of state predicting a
critical universal number depending on the (B,a) moduli space. In the vanishing
limit of the B parameter, we recover the usual perfect gas behavior appearing
in the case of the non rotating BTZ black hole. We point out that in a generic
region of the (B,a) moduli space, the model behaves like a Van der Waals
system.
