  We report a theoretical investigation on the conductance and its fluctuation
of three-dimensional topological insulators (3D TI) in $Bi_2Se_3$ and
$Sb_2Te_3$ in the presence of disorders. Extensive numerical simulations are
carried out. We find that in the diffusive regime the conductance is quantized
with fractional value. Importantly, the conductance fluctuation is also
quantized with a universal value. For 3D TI connected by two terminals, three
independent conductances $G_{zz}$, $G_{xx}$ and $G_{zx}$ are identified where z
is the normal direction of quintuple layer of 3D TI (see inset of Fig.1). The
quantized conductance are found to be $<G_{zz}>=1$, $<G_{xx}>=4/3$ and
$<G_{zx}>=6/5$ with corresponding quantized conductance fluctuation 0.54, 0.47,
and 0.50. The quantization of average conductance and its fluctuation can be
understood by theory of mode mixing. The experimental realization that can
observe the quantization of average conductance is discussed.
