PDF and $\alpha_s$ constraints from jet measurements at CMS

The measurements of the ratio of the inclusive 3-jet cross section to the inclusive 2-jet cross section, and of the double-differential cross section of 3-jet events are presented. The data sample was collected during 2011 at a proton-proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 5.0 $\mathrm{fb}^{-1}$. A comparison between the measurements and the predictions from perturbative QCD at next-to-leading order is performed. Within uncertainties, data and theory are in agreement. Both measurements are used for the extraction of the strong coupling constant, $\ensuremath{\alpha_S(M_Z)}\,$ with the results being in agreement with the world average value. The measurements also serve as a test of the evolution of the strong coupling constant at the 1 TeV region. No deviation from the expected behaviour is observed. The sensitivity of the observables to parameters of the theory like the parton distribution functions is also studied.