This abstract describes a trilinear immersed FEM (IFE) for solving 3-D elliptic interface problems avoiding the sometimes difficult task of generating geometry fitting meshes. The method uses hexahedral Cartesian meshes independent of the interfaces. Interfaces are represented with levelsets and the finite element space is locally modified to better approximate the discontinuities of the solution. The method is applied to solve the EEG (Electro-EncephaloGraphy) forward problem, thus avoiding the complex problem of meshing the head. Numerical results show that the proposed method achieves the same accuracy as the standard linear FEM with geometry fitting mesh.