  We calculate the three-parton twist-3 contribution to the $B\to\pi$
transition form factors in the $k_T$ factorization theorem. Since different
mesons are involved in the initial and final states,
two(three)-parton-to-three(two)-parton amplitudes do not vanish. It is found
that the dominant contribution arises from the diagrams with the additional
valence gluon attaching to the leading-order hard gluon. Employing the
three-parton meson distribution amplitudes from QCD sum rules, we show that
this subleading piece amounts only up to few percents of the form factors at
large recoil of the pion. The framework for analyzing three-parton
contributions to $B$ meson decays in the $k_T$ factorization is established.
