Your post is in LED. 3 AAA could direct-drive an emitter, so it would tail off towards zero output over infinite time. 2 AA requires a boost driver, which will have a particular efficiency and a minimum functional voltage.
AAs usually have 2 Amp hours, while AAAs usually get about 800 mAh. A pair of AAs has 2 times voltage times capacity - for eneloops, 2*1.45v*2 Ah = 5.8 watt-hours. This is approximate, as the voltage will drop slightly towards the end of battery life. 3 eneloops AAAs would have 3 * 1.45v * .8 Ah = 3.48 watt-hours.
You'll get more photon-hours at a reasonable level from 2 AAs, but 3 AAAs will still weakly light for a long time.