8xAA battery cells make 12V only when they are brand new. The voltage will quickly drop to 9.6V as they are used then drop slower to below 8V.
Assuming you want the LEDs to still light (but dimmed) when the battery has dropped to 9V then you cannot have three 3V LEDs in series and in series with a current-limiting resistor.
Another problem is that NOBODY makes a 3V LED (but expensive high power LEDs are tested and "binned" into certain voltage groups).
LEDs have a range of voltages maybe from 2.7V to 3.3V. Some will be 3V but some will also be 2.7V and 3.3V or anywhere in between. Buy thousands of LEDs, test the voltage of each one, sort them then use only the ones (if there are any) that are 2.7V.