  We determine two improvement coefficients which are relevant to cancel
mass-dependent cutoff effects in correlation functions with operator insertions
of the non-singlet local QCD vector current. This determination is based on
degenerate three-flavor QCD simulations of non-perturbatively O(a) improved
Wilson fermions with tree-level improved gauge action. Employing a very robust
strategy that has been pioneered in the quenched approximation leads to an
accurate estimate of a counterterm cancelling dynamical quark cutoff effects
linear in the trace of the quark mass matrix. To our knowledge this is the
first time that such an effect has been determined systematically with large
significance.
