What do you need from M3 that you can't get from M2?
The Fed said the same thing, but it doesn't make any sense as a question. M2 doesn't include large denomination deposits, institutional money funds, etc. Saying M3 doesn't matter because we know M2 would be like saying we don't care about the Atlantic ocean because we have a good idea of what the Mississippi river is like.
Hiding M3 lets them cook the books and get away with it much more easily than if they published the numbers.