  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.
