fbx to txt conversion in the context of plain text format export from the FBX 3D model is not very likely. But several 3D modeling formats are text (ASCII) based, so perhaps this is actually why this export is needed.
One of the most common text-based 3D formats is OBJ for example, but there are many others, so you might need to refine your search based on a specific format to find the most suitable tool for your needs.