We explore the possibility to observe hard exclusive three-jet production in early LHC runs, corresponding to diffractive dissociation of the incident proton into three jets with large but compensating transverse momenta. This process is sensitive to the proton unintegrated gluon distribution at small x and to the distribution of the three valence quarks in the proton at small transverse distances. The corresponding cross section is calculated using an approach based on k(t) factorization. According to our estimates, observation of hard diffractive three-jet production at LHC is feasible for jet transverse momenta q(perpendicular to) similar to 5 GeV. (C) 2008 Elsevier B.V. All rights reserved.