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 ...