We are working with the NanoPC-T4 and NanoPi NEO4 (pretty similar boards). We are running FriendlyCore, and we are able to use the I2C bus on pins 3 and 5 just fine via the exposed /dev/i2c-2 device. Both boards state that pin 8 is "GPIO4_C1/I2C3_SCL", and pin 10 is "GPIO4_C0/I2C3_SDA". However, we don't even have a /dev/i2c-3 device, nor can we figure out a way to use that second I2C bus.

Is there something we need to do to enable I2C in pins 8 and 10? It would be very handy for us to have a second I2C bus.