Combinatorial formula for the $M$ invariant of magnetic lines

To solve MHD problems within the framework of the theory of two-scale mean fields, it is important to study the invariants of magnetic lines. Such invariants are constructed on the basis of invariants of classical links, which must satisfy the asymptotic property. We choose the simplest asymptotic invariant $M_3$ of three-component links, which is not expressed in terms of the pairwise linking coefficients of the components. We check the asymptotic property based only on the combinatorial definition of the invariant and do not use the analytic integral. For simple examples, the proven formula is verified by calculation.