MSP430FR5970: MSP430 ADC Pin Configuration

Tool/software:

Hi,

I'm planning to monitor 12V, 3.3V, and 1.8V, which I'm connecting to ADC pins using a resistor divider and considering VREF = 1.5V.
So for 12V, R1 = 69.8Kohm, R2 = 10Kohm (A0).
For 3.3V, R1 = 5Kohm and R2 = 10Kohm (A1).
This both will voltage divide it into 1.5V reference to ADC pins. How the voltage monitoring actually happened based on this reference voltage? Both the A0 and A1 are fed with 1.5 reference; how is the MCU differentiating the 12V and 3.3V? Also, if the source input voltage reduces, for example, if 12V comes to 11.8V, how will the MCU read it?

Regards,
Anoop