Consider tri-linear forms, $\{A_{ijk}\}$ where $i=1,..,n_1$, $j=1,..,n_2$, $k=1,..n_3$, over a field of zero characteristic, up to the equivalence $A\to (U_1,U_2,U_3)(A)$, by three matrices. What is known about the classification? (Complete) invariants? Unlike the case of bi-linear forms, in general the classification has moduli.

I've found only an old paper of Thrall, dealing with partial cases. Is there some general (and more modern) exposition/introduction/lecture notes?

My particular question: for which triples $(n_1,n_2,n_3)$ is the classification discrete?