A measurement of the ratio of the inclusive 3-jet to 2-jet cross sections as a function of the total jet transverse momentum, HT, in the range 0.2<HT<2.5TeV is presented. The data have been collected at a proton–proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, and correspond to an integrated luminosity of 36 pb−1. Comparisons are made between the data and the predictions of different QCD-based Monte Carlo models for multijet production. All models considered in this study are consistent with the data for HT>0.5TeV. This measurement extends to an HT range that has not been explored before.