  Toward the understanding of incommensurate $\Gamma_3$ quadrupole ordering in
PrPb$_3$, we develop a microscopic theory of multipole ordering in
$f^2$-electron systems from an itinerant picture on the basis of a $j$-$j$
coupling scheme. For this purpose, we introduce the $\Gamma_7$-$\Gamma_8$
Hubbard model on a simple cubic lattice with the effective interactions that
induce local $\Gamma_3$ states. By evaluating multipole susceptibility in a
random phase approximation, we find that the hybridization between $\Gamma_7$
and $\Gamma_8$ orbitals plays a key role in the emergence of $\Gamma_3$
quadrupole ordering. We also emphasize that $\Gamma_3$ quadrupole ordering can
be understood from the concept of {\it multipole nesting}, in which the Fermi
surface region with large $\Gamma_8$ orbital density should be nested on the
area with a significant $\Gamma_7$ component when the positions of the Fermi
surfaces are shifted by the ordering vector. This concept cab be intuitively
understood from the fact that local $\Gamma_3$ doublets are mainly composed of
two singlets between $\Gamma_8$ and $\Gamma_7$ orbitals. Finally, we discuss
the possible relevance of the present theory to the experimental results of
PrPb$_3$ and point out some future problems in this direction of research.
