  A search for charmless three-body decays of $B^0$ and $B_{s}^0$ mesons with a
$K_{\rm S}^0$ meson in the final state is performed using the $pp$ collision
data, corresponding to an integrated luminosity of $1.0\,\mbox{fb}^{-1}$,
collected at a centre-of-mass energy of $7\mathrm{\,Te\kern -0.1em V}$ recorded
by the LHCb experiment. Branching fractions of the $B_{(s)}^0 \to K_{\rm S}^0
h^{+} h^{\prime -}$ decay modes ($h^{(\prime)} = \pi, K$), relative to the well
measured $B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-}$ decay, are obtained. First
observation of the decay modes $B_s^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp}$ and
$B_s^0 \to K_{\rm S}^0 \pi^{+} \pi^{-}$ and confirmation of the decay $B^0 \to
K_{\rm S}^0 K^{\pm} \pi^{\mp}$ are reported. The following relative branching
fraction measurements or limits are obtained \begin{eqnarray*} \nonumber
\frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp})}{{\cal B}(B^0 \to K_{\rm
S}^0 \pi^{+} \pi^{-})} &=& 0.128 \pm 0.017 \, ({\rm stat.}) \pm 0.009 \, ({\rm
syst.}) \,, \\ \nonumber \frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{+} K^{-}
)}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &=& 0.385 \pm 0.031 \, ({\rm
stat.}) \pm 0.023 \, ({\rm syst.}) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to
K_{\rm S}^0 \pi^{+} \pi^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})}
&=& 0.29\phantom{0} \pm 0.06\phantom{0} \, ({\rm stat.}) \pm 0.03\phantom{0} \,
({\rm syst.}) \pm 0.02 \, (f_s/f_d) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to
K_{\rm S}^0 K^{\pm} \pi^{\mp})}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})}
&=& 1.48\phantom{0} \pm 0.12\phantom{0} \, ({\rm stat.}) \pm 0.08\phantom{0} \,
({\rm syst.}) \pm 0.12 \, (f_s/f_d) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to
K_{\rm S}^0 K^{+} K^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &\in&
[0.004;0.068] \; {\rm at \;\; 90\% \; CL} \,. \end{eqnarray*}
