So just to comfirm…
You have plugged in the XM+ to UART 3
In Betaflight on the Ports page:
You have selected Serial RX on UART 3 and nothing else.
Then clicked “Save and Reboot”
Then on the Configuration page:
Receiver Mode - Serial-based receiver (SPEKSAT, SBUS, SUMD)
Serial Receiver Provider - SBUS
Then clicked “Save and Reboot”