FitGirl repacks usually include a tool called Verify BIN files before installation.bat . Once you have extracted the files: Run that batch file.
Select or "Extract to..." using a tool like 7-Zip or WinRAR . Immortals - Fenyx Rising [FitGirl Repack].part3...
The software will automatically pull data from Part 2, Part 3, and so on to recreate the full installer. FitGirl repacks usually include a tool called Verify