We investigate n-component systems of conservation laws that possess third-order Hamiltonian structures of differential-geometric type. The classification of such systems is reduced to the projective classification of linear congruences of lines in P n+2 satisfying additional geometric constraints. Algebraically, the problem can be reformulated as follows: for a vector space W of dimension n + 2, classify n-tuples of skew-symmetric 2-forms A α ∈ Λ 2 (W ) such that φ βγ A β ∧ A γ = 0, for some non-degenerate symmetric φ.MSC: 37K05, 37K10, 37K20, 37K25. 1