  We present results for exclusive $b\to c$ semileptonic decays of ground state
triply-heavy baryons and for semileptonic $c\to s,d$ decays of doubly heavy
ground state $cb$ baryons. In both cases, we have derived for the first time
heavy quark spin symmetry relations for the hadronic amplitudes near zero
recoil. Though strictly valid in the limit of very large heavy quark masses and
near zero recoil, they turn out to be reasonable accurate for the whole
available phase space in these decays and for the actual heavy quark masses we
use. With these relations we have made approximate, but model independent,
predictions for ratios of decay widths. In the case of spin-1/2 $cb$ baryons,
we find that hyperfine mixing in the wave function has a great impact on their
$c\to s,d$ decay widths.
