With all the Raspberry PI 3 I have in production, I need the download link for the SBC for these. I assume it is different then the Raspberry PI 4 but that is all I can find.
You could follow the same guide we have for the Pi 4, just make sure to flash the SD card with the image provided in the guide which is based on Debian Buster: https://www.3cx.com/docs/installing-pbx-raspberry-pi/