  We study the reduced skewness,
$S_{3,g}\equiv\bar{\xi}_{3,g}/\bar{\xi}_{2,g}^2$ 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 $2-3\sigma$ 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.
