We report a measurement of the $B^0$ and $B^+$ meson decays to the $D_s^-K^0_S pi^+$ and $D_s^- K^+K^+$ final states, respectively, using $657 times 10^{6} Boverline{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. Using the $D_s^-to phipi^-$, $K^{*}(892)^0 K^-$ and $K^0_S K^-$ decay modes for the $D_s$ reconstruction, we measure the following branching fractions: ${cal B}(B^0to D_s^{-}K^0_Spi^+)=[0.47 pm 0.06 (mathrm {stat}) pm 0.05 (mathrm {syst})]times 10^{-4}$ and ${cal B}(B^+to D_s^-K^+K^+)= [0.93 pm 0.22 (mathrm {stat})pm 0.10 (mathrm {syst})]times 10^{-5}$. We find the ratio of the branching fraction of $B^+to D_s^-K^+K^+$ to that of the analogous Cabibbo favored $B^+to D_s^-K^+pi^+$ decay to be ${cal R}_{cal B} = 0.054 pm 0.013 ({rm stat}) pm 0.006 ({rm syst})$, which is consistent with the na{i}ve factorization model. We also observe a deviation from the three-body phase-space model for both studied decays.
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