AIPAC, B'nai B'rith & the AJC all use to claim 3 million+ Jews died at Auschwitz. How did they get it so wrong?

That's because they got the whole Holocaust story wrong. At one point, they thought they could make the most outrageous claims without anyone objecting because, after all, they were "victims". It wasn't until later that they started lowering the numbers down to acceptable figures.