  Renormalization Group (RG) and optimized Pade-approximant methods are used to
estimate the three-loop perturbative contributions to the inclusive
semileptonic b \to u and b \to c decay rates. It is noted that the \bar{MS}
scheme works favorably in the b \to u case whereas the pole mass scheme shows
better convergence in the b \to c case. Upon the inclusion of the estimated
three-loop contribution, we find the full perturbative decay rate to be
192\pi^3\Gamma(b\to u\bar\nu_\ell\ell^-)/(G_F^2| V_{ub}|^2) = 2065 \pm 290{\rm
GeV^5} and 192\pi^3\Gamma(b\to c\ell^-\bar\nu_\ell)/(G_F^2|V_{cb}|^2)= 992 \pm
198 {\rm GeV^5}, respectively. The errors are inclusive of theoretical
uncertainties and non-perturbative effects. Ultimately, these perturbative
contributions reduce the theoretical uncertainty in the extraction of the CKM
matrix elements |V_{ub}| and |V_{cb}| from their respective measured inclusive
semileptonic branching ratio(s).
