MSPM0G3507: How to reset SPI Peripheral's receiver shift register bit counter?

Tool/software:

Hi all,

It's about the SPI peripheral's receiver shift register.

I know we can auto clear the shift register bit counter when CS inactive in 4 wires-SPI model.  

The situation is that in 3 wires-SPI model there's no CS, how do we do it? Is it possible to manually clear that bit counter?

Thanks,