With the recent discovery of magnetic 3D topological insulators (3DTIs), new routes have opened for fundamental research on topological materials. In particular, devices made of heterostructures of magnetic and non-magnetic 3DTIs offer promising routes for high-temperature quantum anomalous Hall systems, with possible applications in metrology as standards of electrical resistance (the Ohm unit) which could be more accessible and thus be widely spread in the industry. Still, any complex heterostructure design must rely on the precise control and understanding of material, electronic and topological properties of 3DTIs. Here we propose to characterize the electronic properties of gated devices of locally grown epitaxial 3D topological insulator thin films (BiSb, BiSbTe and BiSbTeSe) in view of further 3DTI heterostructuring, using magnetotransport, Landau level spectroscopy and first-principles calculations at very high magnetic fields (50-70T). This project strengthens and develops a local collaboration between three labs in Toulouse: the LNCMI (transport measurements at very high fields), the CEMES (theory, DFT), and the LAAS (epitaxial growth and nanofabrication). It will serve as a stepping stone for a full 3DTI magnetic heterostructures.