Is it possible to run Raspberry Pi 3 with a 64-bit kernel and 32-bit user space?
Yes. Some pre-built images have this ready to go:
Crazyhead90's Raspbian Stretch Lite and Desktop 64-bit images (Pi 3, 3B+)
sakaki's Raspbian Buster Desktop 64-bit image (Pi 4)
usually running this "hybrid" configuration would break the package managers, but I'm not worried ...
You were almost there with your command. After you download the package, run the following command:
python3 setup.py install
That is how you install a package (you were missing the install at the end).
If you have no internet access, you will need to solve the dependencies by hand (i.e., install ALL the packages required), so you will need to do a bit of ...