Where do i get the latest driver for RAK831 and Rasp Pi 3B ?
Hi. Driver is not what you need 
Here are the latest RPi3 and RPi4 Images for RAK831: https://downloads.rakwireless.com/en/LoRa/RAK831-Pilot-Gateway/Firmware/
If you do not want to burn images to sd card, you can install from here https://github.com/RAKWireless/rak_common_for_gateway
Thanks very much !!!