Any Raspberry Pi which is supported by Raspberry Pi OS (previously “Raspbian”) is also supported by the SatNOGS Image, including the Pi 3.
Yes, the SatNOGS Image is based on bullseye (announcement in this post).
Raspbian Buster is no longer supported by SatNOGS Client Software. Unfortunately it was not possible to implement an upgrade for “old” SatNOGS Stations (based on Raspbian buster) to bullseye. Thus, to update an “old” SatNOGS station (as identified by “DESCRIPTION”: “Raspbian GNU/Linux 10 (buster)” in the Support Information), flashing a fresh image is required (docs).