As commonly done vs As is commonly done vs As it is commonly done

Question

Which of the following three is the correct usage of the phrase?

1. As commonly done
2. As is commonly done
3. As it is commonly done

To me, 3rd one seems to be most appropriate from a grammatical perspective, however I'm not sure.

Answer 1

The second option is more natural sounding to me (as a native English speaker).

The first sounds curt, and comes across as perhaps a little 'lazy'.

The third sounds stilted - the extra word adds nothing, and makes the sentence slightly harder to enunciate. However, this might be used in a more formal situation.

Hope that helps,

Alan.