It is illegal to take fanfics that you find on Tumblr, AO3, from ANYWHERE, and repost them. The authors can pursue legal action if they so choose. “Credit to the owner”/listing the author’s URL name is not justification. It would be exactly the same as putting a full copy of Harry Potter, Lord of the Rings or any other Paper Published book online and then saying “Credit to the Author”. They’d still sue the pants off you. AO3 even has a team of lawyers that will SUE you for stealing fanfiction from their authors.