  A generic formula for the entropy of three-dimensional black holes endowed
with a spin-3 field is found, which depends on the horizon area A and its
spin-3 analogue, given by the reparametrization invariant integral of the
induced spin-3 field at the spacelike section of the horizon. From this result
it can be shown that the absolute value of the spin-3 analogue of the area has
to be bounded from above by A/3^(1/2). The entropy formula is constructed by
requiring the first law of thermodynamics to be fulfilled in terms of the
global charges obtained through the canonical formalism. For the static case,
in the weak spin-3 field limit, our expression for the entropy reduces to the
result found by Campoleoni, Fredenhagen, Pfenninger and Theisen, which has been
recently obtained through a different approach.
