Holomorphically homogeneous CR real hypersurfaces M 3 ⊂ C 2 were classified by Élie Cartan in 1932. A folklore legend tells that an unpublished manuscript of Cartan also treated the next dimension M 5 ⊂ C 3 (in conjunction with his study of bounded homogeneous domains), but no paper or electronic document currently circulates.In the last 20 years, much progress on this classification problem in C 3 has been made. Fels-Kaup classified the 2-nondegenerate hypersurfaces with constant Levi rank 1, obtaining a list that is entirely tubular. For Levi non-degenerate hypersurfaces, substantial efforts due to Loboda and Doubrov-Medvedev-The settled the multiply-transitive case, and the lists are extensive. Recently, Kossovskiy-Loboda completed the strongly pseudoconvex (Levi-definite) simply-transitive case, and all models found are tubular. Only the simply-transitive case with split Levi signature remained.We perform this last study by employing an independent Lie-algebraic approach. A key new geometric tool is a coordinate-free formula for the fundamental (complexified) quartic tensor, which in particular can be specialized to Lie algebra data. Our final result has a unique (Levi-indefinite) nontubular model, which admits planar equi-affine symmetry.While this article was under preparation, Loboda posted his synthetic memoir arXiv:2006.07835, where he also addressed the simply-transitive Levi-indefinite case. We can hence compare and clarify both lists of simply-transitive models.A striking corollary is that all locally homogeneous real hypersurfaces M 5 ⊂ C 3 are now fully classified by these contributions.