  We complete the list of normal forms for effective 3-forms with constant
coefficients with respect to the natural action of symplectomorphisms in
\mathbb{R}^6. We show that the 3-form which corresponds to the Special
Lagrangian equation is among the new members of the classification. The
symplectic symmetry algebras and their Cartan prolongations for these forms are
computed and a local classification theorem for the corresponding Monge-Ampere
equations is proved.
