We report the first low-frequency mechanical relaxation measurement of molecular motions at temperatures near glass transition in side-chain liquid crystalline polymers (SCLCP) with a short alkyl spacer length of 3 in the cholesteric state. The dynamics of molecular motions involved in the liquid-glass transition of side-chain liquid crystalline polymers in the cholesteric state has been studied. The effect of thermal history or physical aging has been investigated. The measured relaxation time when plotted against normalized reciprocal temperature in the metastable equilibrium state shows that the molecular motion exhibits a "fragile" or in other words a"highly cooperative" character. The interpretation of experimental results through two compatible theoretical models suggests that the molecular motions involved in the glass transition are highly correlated in these SCLCPs. Comparison with the data of a similar SCLCP that has a long alkyl spacer length highlights the change in segmental dynamics with alkyl spacer length. In addition, the sensitivity of mechanical spectroscopy makes it possible to measure the variation of the glass transition temperature as a function of the chiral group amount.