Given a null-homologous knot $K$ in a rational homology 3-sphere $M$, and the standard infinite cyclic covering $tilde{X}$ of $(M,K)$, we define an invariant of triples of curves in $tilde{X}$, by means of equivariant triple intersections of surfaces. We prove that this invariant provides a map $phi$ on $Al^{otimes 3}$, where $Al$ is the Alexander module of $(M,K)$, and that the isomorphism class of $phi$ is an invariant of the pair $(M,K)$. For a fixed Blanchfield module $(Al,bl)$, we consider pairs $(M,K)$ whose Blanchfield modules are isomorphic to $(Al,bl)$, equipped with a marking, {em i.e.} a fixed isomorphism from $(Al,bl)$ to the Blanchfield module of $(M,K)$. In this setting, we compute the variation of $phi$ under null borromean surgeries, and we describe the set of all maps $phi$. Finally, we prove that the map $phi$ is a finite type invariant of degree 1 of marked pairs $(M,K)$ with respect to null Lagrangian-preserving surgeries, and we determine the space of all degree 1 invariants of marked pairs $(M,K)$ with rational values.
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