We study the class of 3-dimensional nonlinear 2−hessian equations, where f is an arbitrary smooth function of the variables (x, y, z). We perform preliminary group classification on 2−hessian equation. In fact, we find additional equivalence transformation on the space (x, y, z, u, f ), with the aid of N. Bila's method, then we take their projections on the space (x, y, z, f ), so we prove an optimal system of one-dimensional Lie subalgebras of this equation is generated by A 1 , • • • , A 12 , which introduced in theorem (2), ultimately, A number of new interesting nonlinear invariant models are obtained which have non-trivial invariance algebras. The result of these works is a wide class of equations which summarized in table. So at the end of this work, some exact solutions of 2−hessian equation are presented. The paper is one of the few applications of an algebraic approach to the group classification using Lie method.