In this talk, the structure analysis of the Direct Sampling Method (DSM) in the 3D electromagnetic inverse scattering problem is presented. Even though the DSM is known to be a robust, fast, and efficient non-iterative type algorithm to estimate the support and shape of unknown inhomogeneities from the knowledge of the scattered data in the 2D scalar electromagnetic case, it has to deal with the choice of aproper test polarization dipole since the data are vectorial in the 3D vector case. The choice of this test polarization dipole is a key parameter for the success of the 3D DSM. So, in the following, we carefully analyze the indicator function of DSM using the asymptotic formula of the scattered field under a small volume hypothesis of well-separated inhomogeneities. Thanks to that hypothesis, an analytic formula of the 3D DSM indicator function is established. The already proposed heuristic method to choose the polarization test dipole is theoretically justified, and a new approach is proposed for better efficiency. Various numerical simulations with synthetic and experimental data validate our theoretical results.