CERN-LHC. Measurements of the ratios between inclusive 3-jet and 2-jet cross sections from proton-proton collisions at a centre-of-mass energy of 7 TeV. The data sample, collected in 2011, has an integrated luminosity of 5.0 fb-1. Data are presented as a function of the average transverse momentum of the two leading PT jets and is used for a determination of the strong coupling constant. Details of systematic errors and the statistical correlation matrix are given.