Three terminal magnetic sensing devices (TTMs) having base lead layers in-plane with collector substrate materials, and methods of making the same, are disclosed. In one illustrative example, a collector substrate having an elevated region and a recessed region adjacent the elevated region is provided. An insulator layer is formed in full-film over the collector substrate, and a base lead layer is formed in full-film over the insulator layer and in-plane with semiconductor materials of the elevated region. The insulator materials and the base lead materials that are formed over the elevated region are removed. A sensor stack structure having an emitter region and a base region is then formed over the elevated region such that part of the base region is formed over an end of the base lead layer. A base conductive via may be formed to contact base lead materials of the base lead layer at a suitable distance away from the sensor stack structure. Advantageously, the base conductive via formation may occur without causing damage to the sensor stack structure. Also, the base lead layer is formed in the recessed region of the collector substrate prior to the formation of the sensor stack structure such that the TTM may be entirely in-situ manufactured. Furthermore, the trackwidth of the TTM may be defined directly by the elevated region of the collector substrate. The TTM is suitable for incorporation into nanoscale devices which increase areal recording densities, therefore aiding the revolution in magnetic storage.