We study the reduced skewness, $S_{3,g}equivbar{xi}_{3,g}/bar{xi}_{2,g}^2$ of galaxy distribution at $z=0.5$ in two families of modfied gravity models: the Hu-Sawicki $f(R)$-gravity and normal-branch of Dvali-Gabadadze-Porrati (nDGP) models. We use a set of mock galaxy catalogues specifally designed to match CMASS spectroscopic galaxy sample. For the first time we investigate the third reduced moment of such galaxy distributions both in the redshift space. Our analysis confirms that the signal previously indicated only for dark matter halo catalogues persists also in realistic mock galaxy samples. This result offers a possibility to extract a potential modified gravity signal in $S_3$ from spectroscopic galaxy data without a need for a very precise and self-consistent RSD models constructed for each and every modified gravity scenario separately. We show that the relative deviations from $Lambda$CDM~ $S_{3,g}$ of various modified gravity models can vary from $7$ down to $sim 2-3%$ effects. Albeit, the effect looks small, we show that for considered models it can foster a $2-3sigma$ falsification. Finally we argue that galaxy sample of a significantly higher number density should provide even stronger constraints by limiting shot-noise effects affecting the $S_{3,g}$ estimates at small comoving separations.
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