Data Output
The PX4FLOW module outputs MAVLink packets on USB and serial port. Use QGroundControl to read data from the module. An I2C interface for sensor data reading is offered as well. Third party libraries are available to connect and integrate PX4FLOW data in your projects.
USART3: MAVLink at 115200, 8N1 baud: OPTICAL_FLOW message, OPTICAL_FLOW_RAD message, HEARTBEAT message
USB: Baud rate is not relevant (USB ignores it): OPTICAL_FLOW message, OPTICAL_FLOW_RAD message, HEARTBEAT message, image.
I2C1: latest Flow value (i2c_frame) and accumulated Flow (i2c_integral_frame) values since last I2C readout available for readout.
Therefore, isn’t the USART3 also giving out the same Mavlink packets as the USB?
I think you better check source code to verify the statements. There are inconsistency on what USART should functioning in different PX4flow docs, probably due to partial implementation or lack of update.
So use USB and I2C instead, try to avoid using UART.