  Charmless 3-body decays of B mesons are studied in the framework of the
factorization approach. The nonresonant contributions arising from $B\to
P_1P_2$ transitions are evaluated using heavy meson chiral perturbation theory
(HMChPT). The momentum dependence of nonresonant amplitudes is assumed to be in
the exponential form $e^{-\alpha_{NR}} p_B\cdot(p_i+p_j)}$ so that the HMChPT
results are recovered in the soft meson limit $p_i, p_j\to 0$. In addition, we
have identified another large source of the nonresonant signal in the matrix
elements of scalar densities, e.g. $<K\bar K|\bar ss|0>$, which can be
constrained from the decay $\bar B^0\to K_SK_SK_S$ or $B^-\to K^-K_SK_S$. The
intermediate vector meson contributions to 3-body decays are identified through
the vector current, while the scalar meson resonances are mainly associated
with the scalar density. Their effects are described in terms of the
Breit-Wigner formalism. Our main results are: (i) All KKK modes are dominated
by the nonresonant background. The predicted branching ratios of
$K^+K^-K_{S(L)}$, $K^+K^-K^-$ and $K^-K_SK_S$ modes are consistent with the
data within errors. (ii) Although the penguin-dominated $B^0\to K^+K^-K_{S}$
decay is subject to a potentially significant tree pollution, its effective
$\sin 2\beta$ is very similar to that of the $K_SK_SK_S$ mode. However, direct
CP asymmetry of the former, being of order -4%, is more prominent than the
latter. (iii) For $B\to K\pi\pi$ decays, we found sizable nonresonant
contributions in $K^-\pi^+\pi^-$ and $\bar K^0\pi^+\pi^-$ modes, in agreement
with the Belle measurements but larger than the BaBar result.
