  The general technique of derivation of Dubrovin's equation for the arbitrary
operator pencils is suggested. The question of unique recovering of the
finite-gap potential by coordinates of zeroes of the Psi-function is discussed.
The crucial result of the paper is an autonomous form of Dubrovin's equations
and new trace-formulas for nontrivial spectral problems of the 3-rd order with
trigonal algebraic curve. We show only demonstrative examples, the method is
spread into arbitrary spectral problem including matrix ones.
