The C3D format is described on the c3d.org web site which I maintain. Currently the description is valid but about ten years old. I am working on updating the formal description and will post new documentation by the end of October - there will be a new feature that might affect Blender in a positive way. The new description will document support for UTF-8 encoding for user entered text, typically 3D point labels and names - this does not change the original ASCII definitions or the file format but means that applications reading the C3D file will see weird characters if they do not read the user strings as UTF. The internal parameter names will remain ASCII so internally the files will remain compatible.
There seems to be no more c3d import export plugin in 2.82, where can it be obtained, or has the name changed or under something else ?