  We prove collinear factorization theorem for the process $\pi\gamma^*\to\pi$
at the twist-3 level in the covariant gauge by means of the Ward identity,
concentrating on the two-parton case. It is shown that soft divergences cancel
and collinear divergences are grouped into the pseudo-scalar and pseudo-tensor
two-parton twist-3 pion distribution amplitudes. The delicate summation of a
complete set of diagrams for achieving factorization in momentum, spin, and
color spaces is emphasized. The proof is then extended to the exclusive
semileptonic decay $B\to\pi l\bar\nu$, assuming the hard scale to be of
$O(\sqrt{\bar\Lambda M_B})$, where $\bar\Lambda$ is a hadronic scale and $M_B$
the $B$ meson mass. We explain the distinction between the factorization of
collinear divergences for a pion distribution amplitude and of soft divergences
for a $B$ meson distribution amplitude. The gauge invariance and universality
of the two-parton twist-3 pion distribution amplitudes are confirmed. The proof
presented here can accommodate the leading twist-2 case. We then compare our
proof with that performed in the framework of soft-collinear effective theory.
