If playing a strictly low-luck game, you would roll 1 d6 at 3. Throwing 1 d6 at 3 give the same average results as throwing 3 d6 at 1 (on average, score 1 hit in every other toss of the dice).
Use of low luck rules trades the possibility of lucky dice rolls for the certainty of making hits if enough units are present in the combat.
So yes, you might shoot down three attackers with your AAA if you play by the base rules. But the most likely result is that you will shoot down 1 plane every other time - which is a result you would also get by tossing 1 d6 at 3.
So the answer you are likely looking for is:
If using AAA against three attacking planes, you toss 1d6 with a hit scored for ruling 3 or less.
-Midnight_Reaper
