  CP asymmetries have been measured recently by the LHCb collaboration in
three-body $B^+$ decays to final states involving charged pions and kaons.
Large asymmetries with opposite signs at a level of about 60% have been
observed in $B^\pm\to \pi^\pm({\rm or} K^\pm)\pi^+\pi^-$ and $B^\pm \to \pi^\pm
K^+K^-$ for restricted regions in the Dalitz plots involving $\pi^+\pi^-$ and
$K^+K^-$ with low invariant mass. U-spin is shown to predict corresponding
$\Delta S=0$ and $\Delta S=1$ asymmetries with opposite signs and inversely
proportional to their branching ratios, in analogy with a successful relation
predicted thirteen years ago between asymmetries in $B_s\to K^-\pi^+$ and $B^0
\to K^+ \pi^-$. We compare these predictions with the measured integrated
asymmetries. Effects of specific resonant or non-resonant partial waves on
enhanced asymmetries for low-pair-mass regions of the Dalitz plot are studied
in $B^\pm \to \pi^\pm \pi^+\pi^-$. The closure of low-mass $\pi^+\pi^-$ and
$K^+K^-$ channels involving only $\pi\pi \leftrightarrow K\bar K$ rescattering
may explain by CPT approximately equal magnitudes and opposite signs measured
in $B^\pm\to \pi^\pm\pi^+\pi^-$ and $B^\pm \to \pi^\pm K^+K^-$.
