Using sheets of Nb, Ti and Ta as starting material, NbTiTa superconducting filaments were prepared by extrusion and drawing. The microstructure evolution of the composite after extrusions and the diffusion among Nb, Ti and Ta in the composite were studied. It is found that a large number of crystal defects and the temperature increment appeared, which is caused by the large plastic deformation during hot extrusions, as a result, the Nb, Ti, and Ta sheets diffused each other heavily. The non-superconducting Ti-rich area and superconducting Nb-rich area and Ta-rich area formed during extrusions. The volume percentage of Ti-rich area exceeded 30%, and the transects of Ti-rich area, Nb-rich area, and Ta-rich area all evolved into ribbon-like shape in extrusions and drawing.