Barebones 328p, 2xAA vs 3xAA, which will last longer? So im just running a 328p at 8mhz with the internal oscillator. Datasheet says 1.8 to 5.5v is voltage range.

Will 3xAA in series (~4.5v) last longer than 2xAA in series (~3v), assuming all batteries have the same capacity, theoretically each option should have the same mAh but does the difference in voltage affect things ?
Yes, running at a higher voltage will consume more current.  In the datasheet you have this nice graph:



I have highlighted the two voltages at 8MHz for you. At 4.5V you would typically be using about 4.7mA.  At 3V you would be using around 2.8mA.  That's not taking into account anything else in your circuit of course - just the ATMega itself.