Studies of the Milky Way interstellar medium (ISM) have traditionally relied on two dimensional (2D) sky projections. However, recent advancements have enabled the exploration of the third spatial dimension: distance. Gaia mission, with its precise parallax measurements of over billions of stars, has been instrumental in this progress, allowing for detailed 3D mapping of the dust density in the ISM. Yet, the current 3D reconstructions of the ISM dust distribution are leaving out an important component of the Galaxy: magnetic fields, which are ubiquitous in the ISM. Magnetic fields significantly influence the dynamics of the ISM at various scales, and shape the large-scale structures of the disk and halo of the Galaxy. Despite their importance, our 3D understanding of Galactic magnetic fields remains limited due to the lack of direct, spatially resolved measurements. Over the past two decades, extensive efforts have focused on probing magnetic fields in both the Milky Way and external galaxies through observations of synchrotron and thermal dust emission. However, these tracers only provide LOS integrated information of the plane-of-sky (POS) component of magnetic field, making it challenging to resolve 3D structures where multiple dust clouds overlap. On the other hand, starlight polarization offers a complementary probe. When combined with precise stellar distances information from Gaia, starlight polarization enables tomographic reconstruction of the POS component of the magnetic field, providing a unique means of resolving overlapping structures along the LOS. In this seminar, I will discuss how starlight polarization can be harnessed to reconstruct the magnetic structure of ISM and introduce a new tool we developed to automatically detect thin dust layers along the LOS for tomography, suitable for the upcoming era of polarization surveys.