  Charmless 3-body decays of $B$ mesons are studied using a simple model based
on the framework of the factorization approach. We have identified a large
source of the nonresonant signal in the matrix elements of scalar densities,
e.g. $<K\bar K|\bar ss|0>$. This explains the dominance of the nonresonant
background in $B\to KKK$ decays, the sizable nonresonant fraction of order
$(35\sim 40)%$ in $K^-\pi^+\pi^-$ and $\bar K^0\pi^+\pi^-$ modes and the
smallness of nonresonant rates in $B\to \pi\pi\pi$ decays. We have computed the
resonant and nonresonant contributions to charmless 3-body decays and
determined the rates for the quasi-two-body decays $B\to VP$ and $B\to SP$.
Time-dependent CP asymmetries $\sin2\beta_{\rm eff}$ and $A_{CP}$ in
$K^+K^-K_S,K_SK_SK_S,K_S\pi^+\pi^-$ and $K_S\pi^0\pi^0$ modes are estimated.
