From what I understand, if you have MCU1, you will not receive updates with the 2021.XX.XX.X designation, which is offered on MCU2. On MCU1 YOU will continue to receive the 2020.XX.XX.X designation (i.e. 2020.48.37.2).
I too have a 2017 Model X, MCU1, HW3 and received the 2020.48.37.2 update...