Context : Aims : The Scutum complex in the inner disk of the Galaxy has a number of young star clusters dominated by red supergiants that are heavily obscured by dust extinction and observable only at infrared wavelengths . These clusters are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy . Methods : During the technical commissioning and as a first science verification of the GIANO spectrograph at the Telescopio Nazionale Galileo , we secured high-resolution ( R \simeq 50,000 ) near-infrared spectra of five red supergiants in the young Scutum cluster RSGC3 . Results : Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure , we were able to measure several tens of atomic and molecular lines that were suitable for determining chemical abundances . By means of spectral synthesis and line equivalent width measurements , we obtained abundances of Fe and iron-peak elements such as Ni , Cr , and Cu , alpha ( O , Mg , Si , Ca , Ti ) , other light elements ( C , N , F , Na , Al , and Sc ) , and some s-process elements ( Y , Sr ) . We found average half-solar iron abundances and solar-scaled [ X/Fe ] abundance patterns for most of the elements , consistent with a thin-disk chemistry . We found depletion of [ C/Fe ] and enhancement of [ N/Fe ] , consistent with standard CN burning , and low ^ { 12 } C / ^ { 13 } C abundance ratios ( between 9 and 11 ) , which require extra-mixing processes in the stellar interiors during the post-main sequence evolution . We also found local standard of rest V _ { LSR } =106 km/s and heliocentric V _ { hel } =90 km/s radial velocities with a dispersion of 2.3 km/s . Conclusions : The inferred radial velocities , abundances , and abundance patterns of RSGC3 are very similar to those previously measured in the other two young clusters of the Scutum complex , RSGC1 and RSGC2 , suggesting a common kinematics and chemistry within the Scutum complex .