There are many important details to keep in mind.
You will probably use the 5V of the Arduino board as the reference for your ADC (AREF). Arduino Uno can work with Vin at least 6V.
In this case, the minimum battery voltage that you can measure will be about 6V (3V on each battery). At a lower Vin voltage, the AREF voltage will begin to decrease and the measurement error will be large.
You should also know the minimum allowable battery voltage.
If you want to "suck out" more from your batteries, for example up to 2.75V on each battery, you need to use a different AREF source with lower voltage or internal source 1.1V, but that's another issue.
Try looking for an answer on this forum, in the Arduino reference section Serial, or open a new thread with this question.