Can't open a tar.7z file

I have a file, 200_GB_of_Books_about_Computers_computer_book_ 3rd Pack.tar.7z , and three file parts. If I open 200_GB_of_Books_about_Computers_computer_book_ 3rd Pack.tar.7z I get the error "The archive is empty or Ark could not open its content." Combining the file parts with cat gives me the same error.

Does anyone know how to solve this?

Files are below:
https://terabox.com/s/1qh1tnSP6kHRkmn7SzgLDgA