they all don't

[navi tasan]

1) They all don't do that.
2) All of them don't do that.

Do these mean:
a) None of them does that.
or:
b) Not all of them do that.

I think '1' and '2' are both ambiguous, but their style isn't really good, although they are not incorrect.
I think:
3) They don't all do that.
means 'b'.

Gratefully,
Navi.

 

[Tarheel]

"They don't all do that" is fine, but I despise the other two.

Instead, say:

None of them do that.

 

[Tarheel]

You could also say:

Not all of them do that.