CERN-LHC. The $\chi_{b1}(3P)$ and $\chi_{b2}(3P)$ states are observed through their $\Upsilon(3S)$ $\gamma$ decays, using an event sample of proton-proton collisions collected by the CMS experiment at the CERN LHC. The data were collected at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 80.0 fb$^{-1}$. The $\Upsilon(3S)$ mesons are identified through their dimuon decay channel, while the low-energy photons are detected after converting to e+e- pairs in the silicon tracker, leading to a $\chi_{b}(3P)$ mass resolution of 2.2 MeV. This is the first time that the J=1 and 2 states are well resolved and their masses individually measured: 10513.42$\pm$0.41(stat)$\pm$0.18(syst) MeV and 10524.02$\pm$0.57(stat)$\pm$0.18(syst) MeV; they are determined with respect to the world-average value of the $\Upsilon(3S)$ mass, which has an uncertainty of 0.5 MeV. The mass splitting is measured to be 10.60$\pm$0.64(stat)$\pm$0.17(syst) MeV.