  In this work, we provide a means to quantify genuine tripartite entanglement
in arbitrary (pure and mixed) continuous-variable states as measured by the
Tripartite Entanglement of formation -- a resource-based measure quantifying
genuine multi-partite entanglement in units of elementary
Greenberger-Horne-Zeilinger (GHZ) states called gebits. Furthermore, we predict
its effectiveness in quantifying the tripartite spatial and energy-time
entanglement in photon triplets generated in cascaded spontaneous parametric
down-conversion (SPDC), and find that ordinary nonlinear optics can be a
substantial resource of tripartite entanglement.
